ah yes, the 64 billion dollar question.
that's really what it all boils down to.
The depth and scope described are impressive. 2d turn based means you won't have to spend all your dev time on graphics, allowing you to concentrate on the simulation behind the paint job ( I personally am of the school of thought that "PICTURES are for looking at, GAMES are for PLAYING!").
best of all, you already have the "rules of the game", and just need to implement them in code.
Well, I haven't gotten into audio, really. And in that matter I feel detail is important. I intend to put a LOT of effort into the soundtrack, and use the music to help tell the story. And yes, that is possible, even in games and even with 16-bit sounds. Go play Final Fantasy VI, the music in that game is 16-bit and not only sounds incredible it does a better job telling the story than the dialogue does. (Think "Peter and the Wolf" with visual aids.)
so its all about how many copies you can sell.
the title sounds niche to me. not mass market like angry birds or something. much more hard core. basic demogaphic: turn based 2d rpg fans, especially the hard core ones who want more of everything (like 16 PCs under your control and a max party size of 32, 20 classes, etc).
And tabletop fans, as it is based off a tabletop game. And even if it's priced at $5 and we lose half to royalties we'll only need to sell a couple hundred to cover expenses. Any more than that is profit. This game has a lot going for it in the hard core community, and the world is visually distinctive enough to catch eyes.
i'd start by checking out the competition (if any). those are the titles you'll be competing against for the user's dollars. you'll have to clearly outdo them all. at the same time, try to get a feel for the popularity of the genre based on things like web presences and download counts of similar titles, and number of hits on relevant Google searches, etc.
you will discover one of two things:
1. the market isn't big enough to make it worth your while
2. you could own the market, such as it is, if you build and keep updated a superior product. this may not make you rich, but it might be worth your while. based on the tight focus on hard core turn based 2d rpg, i don't think this will do well enough to allow you to quit your day jobs. 3d has more mass appeal. realtime has more mass appeal. non-hardcore has more mass appeal (unfortunately).
off the top of my head, i'd say you have perhaps as much as a 50% chance of this being something you could make some money at long term. which is much better odds than most titles people consider.
the next step is market research. look at everything out there. get a feel for the current state of the market for this game type. is anybody into these games anymore? whats the competition like? can you outdo them in all respects? or at least outdo them in most respects, and match them in the rest? If there's signs of interest in the game type, and the competition is not too stiff, you may have a winner!
i wouldn't worry about chrome like different graphics for the PCs depending on their age. there will be time for that once you've built the basic game, released it, proven it sells, and is worth the extra time on graphics. See, right there, you already have the beginnings of a feature list for version 2.0, and thus another "franchise" is born. if the game sells, by version 3 or 4 your game should be synonymous with hard core 2d turn based rpg play.
To be honest, we don't need to make a living off this game. We just need to make a profit. I'm not really doing this for money and I'd run it at a loss if I had the means to do so, but I can't afford it. As long as it goes for more than we put in, even by a dollar, that means it's not impairing my standard of living and that's enough for me to make another. Of course, I would like to make money (and since Steven is much more materialistic than I am, he won't stay on if we don't) so I'm planning it to make money. As a result, we needed a couple gimmicks. These include giving it a modding kit so people can make their own campaigns. (Tabletop ruleset means tabletop players. Co-op means people to play with. See the direction this is heading?) Add on small, cheap, frequent DLC. It's a win/win for me. DLC isn't free, so I get money. It also stirs a modding community like nothing else, and as a modder I do enjoy modding communities and the work they put out.