whilst the aim of the game is to be the best thief, there'd be more to it then just completely heists
Actually concentrating on complete heists might work better than adding other stuff. If you remove fighting monsters and stuff, and concentrate on stealing it might turn out to be interesting. The Thief series only used shadows as the element of gameplay that allowed you to be stealthy. But if you think about all the movies about robberies and heists out there as well as all the other games that employed stealth as a component of gameplay, there's a whole lot more ways to steal something than just moving in the shadows, plus treasure isn't the only thing that can be stolen, so for the original idea of having stealing as the aim there's quite a wide field to explore if you just concentrate on it and try to deepen the gameplay.
Yeah, in my original design brief for the thief game, I had this idea of accepting a heist and then being completely open on how you complete it, such as allowing the player to hire individuals to cause a commotion outside the objective allowing you to sneak in, or stealing a uniform for the place you wish to sneak into and blending in.
I didn't really include this because I wasn't sure about the technical aspect to it all. I'm looking at some of these replies and I feel I've done a real poor job of explaining the idea out XD.
My inspiration for this idea has come from so many different places so it's sort of clashed into an unruly mess. I'm talking the Museum level in Uncharted 2, Oceans Eleven, Oblivion Thieves Guild things like that.
For your thief RPG idea, you have next to no explanation on what the RPG elements would be. Additionally, you are willingly opening a huge can of worms by allowing the players items to be stolen by other players. The only way to truly do this 'realistically,' would be to go the CCP route as they allow stealing, destruction, etc within their game of Eve. You would alienate many people, however. For every devout player that Eve holds onto, there are just as many if not more than ragequit over griefing. Know that you would be opening the door to the same things. Not necessarily a bad thing, but still something that will heavily impact your design as well as any viability you have to get your game out there.
It's because I'm wrongly branding the idea as an RPG. I mean, I was thinking just having a skill based system to improve things like stealth, but the more people have commented on it here, the more I think that's not really a good idea and that it'd actually ending taking from the game rather then giving to it. I imagine stealth gameplay is going to be an extremely difficult beast to overcome, because you'll have to juggle player friendly-ness with boredom.
As for the theft of players goods online, the only reall counter idea to that I had was hiring guards for your base but even then I do realize that's not really a viable idea.
It's kind of why I'm posting this here though, to discuss ideas and possibilities

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Lastly, CORPGs, even independent ones, are expensive and hard to create. If all you're looking to do is write up some game design docs to prove you can do, then go ahead, but if this is actually something you would want to produce, I would either make it very contained and single player, or put it on the backburner until you have access to a bundle of resources.
Lastly, copying other games is not bad in itself. Imitation is the best form of flattery, after all. A Thief style CORPG would be a pretty neat idea. Done well, I would definitely want to check it out, but it would be very hard to do a good thief-style CORPG on anything but a AAA budget. Not impossible, mind you... just hard.
Yeah, there's no real intention to make these two games as of yet anyway. I'm currently working on one project with someone from this forum, I just like writing up game design ideas and getting feedback on them. If I get something that's proven to be hugely popular I may look into working on it more so. But yeah, it's literally just to discuss game play mechanics and to also help me improve my understanding of game design/development and the like.
I'd rather have people rip my ideas to pieces if they're crap so that I can learn from my mistakes with them and maybe even refine them.
As for your space idea:
Choice of Scenery; Planet (Desert, Water, Jungle, Metropolis), Asteroid, Space Station (Different planet backdrops).
---Okay, this could work, but what exactly type of manager sim are you wanting to create? Is this Sim City in space? Or is it more of a text-based game, like the older games in the *insert sport here* Manager series, but applied to a science-fiction civilization?
For this idea I was thinking of a smartphone game, game play similar to that of "Game Dev Story" and "Hot Springs Story". But I've sort of messed it all up by adding too many ideas and sort of trampling on my original idea.
So too answer that question, I guess it'd be a mix of the two? You'd have a list of docking ports where you can instruct ships to dock, it'd be graphics based you'd see the ships come in and dock, and you'd be able to see the layout for your space port to place shops and the like. But you'd also have the management aspect for all the economy managing and what not.
Build trade services around your space station such as shops, mercenary camps, freelance miners.
---Sounds exactly like Sim City, but in space. Again, imitation and flattery and all that, but you should remember that Alpha Centauri already did this, and quite well I might add.
Create fleets and send them out to mine planets or asteroid belts for minerals too sell on your markets. Also create trade fleets too send your markets off to other parts of the galaxy to increase income.
---Now the idea is starting to sound like Age of Empires meets Sim City in Space. Additionally, if you are planning on a mostly text-base game, these features could work. If you wanted a full graphical game, however, you are looking at a lot of headaches trying to get this large amount of features into a game on a shoestring budget (as I doubt you would be coming to us for advice if you had access to a AAA budget).
Manage economies befriending empires to increase business (such as, befriending the human empire which will decrease how much the fuel costs them but increase resource yields and fame, and reduce the cost for repairs and upgrades for the player).
---Economy is one of the hardest, bar none, features to implement in any game. While it's easier in a single player game, because you don't need to balance for a cooperative element, it is still an utter nightmare on the scale that you would want to do it. Not impossible, mind you, but don't expect to keep all your hair.
Easy to use UI
---easy to use UIs are always a good design decision. So nothing to really say here.
Own multiple space stations and buy out competing businesses and their space stations and fleets.
---I really think you're pushing the limit on what you can do here. You already have mechanics in which you build businesses, an economy that allows you to build economic alliances (ala Civilization), as well as mechanics allowing for fleet building, resource gathering, etc. Adding in a feature to be able to go all Corporate Mogul on competitors may sound like a simple step, but it's not. This would be almost like an entirely new game in itself in terms of the amount of design and balancing it would need.
Time System including Day and Night system for planetary scenery.
---Could be done. Wouldn't be incredibly easy, given that it's in space, so you would have to have separate cycles for each planet unless they were all in the same system.
Protect your Space Station and fleets from terrorist attacks and alien attacks.
---Now you're adding a combat mechanic and all I can envision is a picture of you beneath a giant pile of rocks, each labelled "features," your arm desperately reaching out from the bottom, crying, "help!"
RPG elements
---And now an elephant has been dropped onto that pile of rocks. How would you even implement RPG elements? In a space-sim, that would be incredibly hard unless you're trying to create a single-player Eve, in which case, stop right now because that's not happening unless you have about $12 million and a team of at least 25 people behind you.
Before I was talking about how I liked to learn about game design and development and what not, well in posting these two I've learned that I'm way too keen when it comes to features in a game

. So yeah this has been discussed quite a lot in this topic and I've felt the best action for this problem is to split the idea into two separate games really.