That is a possibility, or I could tell him that the next game we make together can be as demonic as he likes...

The hard part is the personal message it sends, he knows better than me on most of these things, he is trained in creature design, and so on, so if I tell him no despite all that my worry is that he will take it as me simply not respecting him. The only way I can think to explain it is that it simply isn't part of the vision I want for this game...

Yeah I'm hoping we can meet somewhere in the middle (but still more my side...)

We have each put together a big pile of reference art, so tomorrow we will sit down together and look at each others to come to some agreement. I just worry that I will make my point in a way that will make him feel dismissed. Thank you for the advice.

that simply is not what I want

The problem is that this is a 50/50 split project, which it should never be.

Having had my own experience, and speaking with a number of indie developers (successful and others) it's important that in one way or another, your game has some form of director. Ultimately, you need someone to be the tie breaker, no matter how small/large the team is.

That doesn't mean there shouldn't be a discussion on such things, but there should also be an understanding that should things hit a stalemate, there's a contingency.

Coming in hot with both parties hoping to make this 'their game' ultimately will inevitably to discussions such as these, and until someone has been defined to have the last word in certain matters, this will be a problem.

Do note that you may not need to have one person have the last word on everything. For example, in my team, everything 'creative' falls onto me, even though I'm not actually producing much of that content lately, and everything technical falls onto one of my partners. We have a 'third wheel' in which does not have final say in anything, but is contributing significantly to the project as well.

Find a solution that works for you, but make sure it is clear between your partner and you who has final say on what, and you will solve not just this discussion, but many others to come without hurting each other's feelings. I would suggest presenting the discussion to him by appealing to his logic and explaining how this stalemate is detrimental to the project as a whole, and see how you'll go about breaking down this stalemate into one or various 'director' roles.

Do note that this may end up with you losing creative control over the project, and the game having demons. This may not be what you like, but it may be a concession you'll need to make. At that point, you'll be able to explain your rationale and try to find concessions within how Demons are to be treated, but if he ends up with creative control and determines Demons are a necessity, you'll have to live with it. Can you? If not, how can you expect him to live without?

Either start paying him, or let him have a say. But jokes aside...

If you are extra convincing and have good points about why you both should go with your vision, give it another try.

If he cannot be convinced, then negotiate. Don't try to "force" him to obey your vision, you will only drive him off, which is the last thing you want.

1. What do you rather have, a game that is not following your vision exactly but gets done, or a game never finished?

2. Why are you so sure your vision is better without his additions? Did you ask for a third opinion? Someone you both trust?

3. Are you sure this is still about "what is better for the game", or is it already about "this is not my vision anymore"? If it is the latter, you might have gotten too attached to an idea to let it go into the wild, or have other people work on it... and chances are you have lost the ability to see what is actually for the better because of your fixation on your idea. No offense, but consider the possibility.

When a team of people work on something, they all bring their own ideas into the end product. Usually this is for the better, as two brains come up with birghter ideas than one. Of course this can also mean two brains have completly opposite opinions of something. That can also be a good thing if you use it as a chance to really dig into the differing opinions and try to find out why you both pulling in a different direction.

As long as can accept a compromise, or the fact that you might be wrong, and your artist friend can also, both of you and your project can only get better if you allow an open, and openended discussion on this.

All good points above, also try to consider what parts of the game design are you in charge of? You said your in charge of design and programming. That alone is a large chunk of how the game works, plays and feels. If you also want creatures and art following your vision, then what is he in charge of? Does he have any creative input?

Just from your short description above, it sounds to me like demons would fit fine if your doing a dark/evil theme. Demons fit in fine with undead and he's right that diversity would make it more interesting.

You have some wiggle room for complaint if demon-summoning won't fit into the existing mechanics. Say for example, to collect a monster you need to dig up a grave or kill a living one or something. Now, if you add demons, that mechanic won't make any sense and you need to add a summoning mechanic, which now means he's indirectly telling you to change what you're doing since you have to code a new mechanic. If this is the case, then you've got a good grounds for refusal, especially since it will complicate development and push back your deadline.

I say if adding demons will not extend the development time, change mechanics, require a new level, and it will still look good and make sense- then consider it. But, if you need to add fire-effects, do a level, make new summoning mechanics, etc etc, it's going to take longer and complicate development. At that point, I'd say no. If this is the case, then consider it for a DLC, or put it on a "wish list", which you can work on after the core game is completed.

Also consider the artist perspective and remind your friend - that it's normal for good artists to get bored of their work. He'll eventually go from bored of it, to hating it. That's why artist are artists. Artist crave novelty, which drives them to improve. With a game, you're prepetually stuck looking at 'trash art' you made a year ago, which you could remake even better or expand upon if you ONLY had time. I constantly have this problem with my designs. I have to dumb-down my style and limit my creativity to have any chance of finishing my project. I'm to the point where i absolutely hate the style and design, but i keep getting compliments about its look. So.. I do the responsible thing and stick with it.