The OP did ask about money, but I agree, getting a boxed copy of a brand new game would be awesome.
If I were to design a game for the Amiga I'd keep it open source. Open Source doesn't always mean free as in beer though, so we (the project members) would be able to charge for it if we wanted to.
Granted I can't code worth a damn right now, but am wanting to learn. But for this discussion, let's assume that we're all together on a game that eventually we're going to release for 10 Euros.
We'd have to decide the features, what the game's objectives are, what sort of artistry we'd need (including sound, which is just auditory art), etc.
Then we'd have to decide if our goals are possible on a stock A500. Do we use that as the base, and then if someone happens to have an A1200 add an AGA mode that looks nicer? Or even forgetting graphics, but think about processor speed for AI, and larger maps, etc. A good example would be comparing the Ultimas and how each of them evolved over time.
Ultima 1; Had rather huge maps, one square being = to one food, dungeons and towns you could enter were all over, and it had (if I recall) four islands, and that's where the load screens were.
I didn't play Ultima2, but Ultima 3 had a full blown world where you just wandered (at least the Amiga version I just tested. I can't recall on the ol' 8-bit where I first played it)
Ultima 4; It was splint up into sectors where it would load when you went into the next one, but would cache the previous one (there was a cheat where you could flip the disk over, and it would load the sector it was supposed to, but the land would have random things on there, if you did it in the right spot (near Britain) you'd get a free boat!)
Ultima 5 was the same as Ultima 4, but Ultima 6 was a fully zoomed in map, down to the character level, so you didn't have single icon entrances for towns. It was massive. It was also the last Ultima to be released on the Amiga. This also managed to get ported to the C64, and by all accounts I've read about it, it was HORRIBLE. Disk swapping all the time, and they removed the triple crossbow!
Ultima 7 is thought of by most to be the best one, and it could have been done on an AGA Amiga quite easily. (in Fact NovaCoder is working on updating the Exult port for that very reason, hopefully it becomes playable, because I never beat that one and would LOVE to play it on the Amiga!)
But the point is as games have gotten more complex, they've needed more hardware. The problem of current games is that they've lost complexity (and fun) because they're all about how many dynamic lights can their engine handle, and how many levels of anti-aliasing one can achieve on pixels that the human eye can barely see.
I think that's why the 'Indie' game scene is taking off so much now, because older people are remembering how awesome games like Superfrog and Turrican were. Before this new group of games, the last really awesome 2D platform game I recall coming out was Rayman. Which is another excellent game that an A1200 could pull off smoothly, but I think an A500 would be really hard pressed to do so.
So even if a design team was formed, if there was a need for an OCS/ECS version of the game running on an 68000 with 512kb of RAM, there would certainly have to be sacrifices for it, which may or may not be worth it.