I am currently having an argument with another designer on a project I am working on. Before I can go into the details of the argument, I have to tell you all a little bit about the project. Essentially the objective of the game is to survive as long as possible in a small 2D playspace with balls of various physics and abilities bouncing around. If the ball touches the player then they die. To help mix things up we also have it so that the levels each have their own special property that makes them more dynamic and have an effect on gameplay. An example for a ball physic would be it curving like a curve ball. An example for a level property would be low gravity. Now on to the debate. On top of what we already have the other designer would like to have different characters with special abilities of their own. I think it would make things too complex and making sure each physic, property, and ability works well in relation to each other would be a pain and would drastically effect level design (it may not seem obvious at first, but with some of the specific traits of things it would). He justifies his case by saying he believes video games are all about empowering the player and the more choice the better. He also says it would add longevity to the game by adding more varied gameplay, though as I stated before I think it would be over-complicating what was supposed to be a fairly simple and straightforward game. What do you think? Do you think that empowering the player with more choice over their game is worth making things more complicated than they should be, and even risking being over-complicated?
Choice in Games
I wouldn't say more choices are always better, but I do like to have *sufficient* choices. (From your description of the game, I think I might be on the side of "yeah, I'd enjoy a little more choice", but not having played it I couldn't say. I'd say treat it as an empirical issue rather than a philosophical one. Finish a few mid-game levels and get them in front of independent eyes.)
My suggestion, though, is that you could have it both ways. Design an experience around your original character, and then put the others in whether or not it "works". Make no pretense of balance; just let it be what it is, and give the player the information to let them choose what challenges they will. On character selection say "97% of players could complete this stage as Wonderman, 72% could complete this stage as Skelly, 15% could complete this stage as Lumpy, 3% could complete this stage as Granny."
Another possibility: have the other characters be temporary power-ups (like instead of a ninja character, have a ninja suit powerup). The power might be better suited for that stage (which is ok, it's a powerup) or it might be disastrous (which is also ok; it becomes another thing to avoid).
I had thought about making it a power up myself but my fellow designer do not like the idea.
Two people with absolutely equal say making decisions with zero tie-breaking capabilities can lead to complications. This is why there is usually a 'lead' mechanic designer and a 'lead' visual designer and a 'lead' sound designer, and so on. 
The two of you may want to figure out a way to solve creative differences like these prior to them occurring in the future, instead of coming to an online forum to rally support for *your* view over your partner's view (which may result in slight bitterness on whoever doesn't 'win' their viewpoint).
On top of what we already have the other designer would like...
"On top of what we already have... the other designer would like..."
Way to be unbiased in presenting both sides equally. 
I read your statement as, "Fact: We already have alot of stuff, and we shouldn't add any more. The other designer has emotional and irrational wants, that I oh-so-rationally am trying to dissuade him of."
I think the real issue isn't this tiny debate over a tiny game design question, it's a bit of dismissiveness towards your partner's ideas. My apologies if I'm missing the mark here - it's hard to pick up relational and emotional issues over a text medium, but they definitely play a major part in partnerships. 
I also feel like (again, from a lack of information) your team is lacking leadership structure - two 'head' cooks can't keep adding salt to the dish, or the dish will get ruined. There needs to be one head cook. Maybe this time it should be your friend, with the two of you discussing ideas, but then you taking the low road if the two of you encounter irreconcilable creative differences? I mean, it's a minor project overall. And then maybe you lead the next project?
He justifies his case by saying he believes video games are all about empowering the player...
That is definitely one major school of thought, propagated by one of the most famous game designers.
"A game is a series of interesting choices." - Sid Meier
[Interesting Decisions - Game Developer Conference Vault]
Extra Credits:
Choice and Conflict
The Feeling of Agency
Videogames and Choice
I don't necessarily hold to it myself as the most important, but I definitely believe the importance of player choice shouldn't be understated.
I don't advocate jamming choices willy-nilly ontop of existing game design decisions, but I'm going to assume that your design partner is not acting without thought.
...and the more choice the better.
I disagree that the more choice is always better. You don't want to put so much choice before players at one time, that they get struct tharn* by option overload. This is why games generally have you unlock options as you progress - one, as a way to reward you and keep you interested by new features, but also so you learn each option/gun/mechanic/choice as you play, without being lost in a plethora of too much to do at once. However, alot of choice is usually good. The primarily thing is how you present the choices and how many you present at once.
*frozen in terror or stupefied by indecision in the face of danger or the unknown. e.g. a deer caught in the headlights.
He also says it would add longevity to the game by adding more varied gameplay, though as I stated before I think it would be over-complicating what was supposed to be a fairly simple and straightforward game. What do you think? Do you think that empowering the player with more choice over their game is worth making things more complicated than they should be, and even risking being over-complicated?
Depends on the amount of complexity. I think adding some choice to an otherwise simple game may increase replay value and may actually make the game more interesting. If you started with three characters, and unlocked two more later, then it wouldn't be overcomplicating things for the player. Especially if the differences between the characters is mainly cosmetic.
Again, this is a small game with the primary goal being entertaining for the two of you to develop. Maybe it'll be entertaining for him to add more choice to the game?
Will I have any real incentive to use these "cool features" in a realistic situation (determined by the game mechanics of the game?
When it comes to good design of anything, you should to have a solid goal, and you should to make purposeful and "thoughtful" design choices.
It seems that there was no clear and agreed upon agenda from the beginning. Two opinions are an unbalanced scale, so good thing you asked here.
Is there a compromise that can be made? Or is it absolutely absurd, your co-worker's idea? I think a compromise can be made.
I had a similar problem with the edutainment tower defense game I was working on some months ago. By the time I hit the second milestone, I had ... some bad feelings about it.
He justifies his case by saying he believes video games are all about empowering the player and the more choice the better. He also says it would add longevity to the game by adding more varied gameplay, though as I stated before I think it would be over-complicating what was supposed to be a fairly simple and straightforward game. What do you think? Do you think that empowering the player with more choice over their game is worth making things more complicated than they should be, and even risking being over-complicated?
Similarly as you, I considered adding multiple layers of complexity to let the game last longer. I decided to halt the project for a bit so I could re-read the GDD and elaborate it a bit. I still had a bad feeling. I left the project alone for a month so I could detach from it.
Interesting conclusions you came to there Servant of the lord. The ones in regard to my relationship with my fellow designer are false, but interesting nonetheless. I value his opinion as much as my own, after all he has spent just as much time working on game design as me, but he tends to get... what's the word I'm looking for... clingy. He can't seem to let go of concepts very well. making it hard to iterate on them, which is something I really like to do. I like to question what we have already come up with and to think "how could we do this better" or "do we really need this the way it is" because I believe to come up with the best product possible you have to ask those questions. This is probably where most of our tension is created. Once Kurt, my partner, comes up with a general idea for what he thinks the game should be, which is usually already pretty good but still, that is how he likes to leave it. He is more of a perfect the initial concept guy and I'm more of a question and change the initial concept until we get the "best" possible game. This directly effects this situation because of what I am about to tell you. Kurt more or less went entirely out of contact with the rest of the team for almost a month. Naturally I have to keep on working without him at that point so I thought "Hey, let me take a look back at what we got and ITERATE", since we actually never did too much of that after the idea for the game was generated. I wasn't changing the core gameplay as much as the stuff built around it. Originally we had different characters with different abilities, but along the way that got scrapped (there were several reasons behind it but I'm not going to write them all out). So yesterday Kurt got back in contact with us and we went back to work. Now we are looking at what we had before and the changes I proposed and we are working on crafting a game that is truly both of ours. However there is this one gleaming issue which both of us can't seem to come to an agreement on. Believe me, I have no issue with having the character abilities but I think they need to be implemented differently. So I guess I should change the question at this point from "who's right, him or me" to "how can I implement the abilities in a unique way that is also not always present while playing the game". I know one guy suggested power-ups and we looked at that, but we aren't sold. I'm sorry if I came off as hostile or disrespectful of my partner Kurt in the original post. No harm was intended.
