I dislike the idea of competing with AI heroes. I don't mind the idea of other heroes being in the game, but I want to explore the world, not dash through it. If you create an awesome world, why not let me take the time to appreciate it? If you create X amount of content, why force me to rush through it faster?

I like the idea of it being in a single open-world town, and I like the idea of the bat-hero and think that'd work great in that environment.

I think it'd be especially cool and fit the theme if the entire town was in a gigantic underground cavern, including buildings and such that have grown up the walls of the cavern, for you to basically climb real high and then glide throughout the cavern, and using that to reach all kinds of secrets.

You didn't need to mention the engine. That isn't game design.

To clarify: engine might affect design choices, but you didn't ask about that (you just threw in information that wasn't related).

For example, game design issues include zoological issues. A "realistic-looking" bat character doesn't seem suitable for "standing on two feet" (bats ordinarily hang from above, mostly head down), or "moving as fast as Mario" (it might cautiously crawl on all fours), and it certainly can't "punch, kick and butt-stomp" effectively with its light frame and delicate little claws

But above all, why would a bat accept to confront a platformer game on its own terms, walking, jumping and exchanging blows, instead of flying and attacking in flight?

For example, game design issues include zoological issues. A "realistic-looking" bat character doesn't seem suitable for "standing on two feet" (bats ordinarily hang from above, mostly head down), or "moving as fast as Mario" (it might cautiously crawl on all fours), and it certainly can't "punch, kick and butt-stomp" effectively with its light frame and delicate little claws

But above all, why would a bat accept to confront a platformer game on its own terms, walking, jumping and exchanging blows, instead of flying and attacking in flight?

Add to that, flying and attacking in the dark of the night where the bat has the edge over its opponents (whereas in bright sunlight, the bat will most probably be an easy prey for most predators).

Sounds like trying to put the square peg into the round hole.

Even if you go all Antropomorphic on your bat (which goes against "realistic"...), your character still should have at least SOME of the qualities (read strenghts/weaknesses) of a bat, and your game design should make it so players are either playing to the strengths of the character (the character can choose the rules of engagements and thus has the upper hand), or clearly have to play against them (because the rules have been forced upon our poor hero)... in the second case, make it clear that your hero has to struggle (like a bat having to fight in bright sunlight, or being forced NOT to fly), and use it to your game designs advantage.

Add to that, flying and attacking in the dark of the night where the bat has the edge over its opponents (whereas in bright sunlight, the bat will most probably be an easy prey for most predators).

Yes and no. Seeing in the dark to allow playing the game would be a reasonable artistic license, and it could be plausibly presented with stylized graphics as sonar range measurements.

Alternatively, the environment can be actually well-lit: a bat is only disadvantaged in daylight against enemies who fly better than he does and can catch him, or against enemies who can shoot him down, or against preys who can see it and run away, None of these categories needs to be a serious problem; many creatures remain too slow and grounded to put up a fight with a bat.

I would.only consider a bat character in a flight-based game, such as a first person or third person 3D flight simulator with explicit control of sprints and swoops and tongue lashes, or a 2D shoot'em up with close-range attacks and little shooting.

Yes and no. Seeing in the dark to allow playing the game would be a reasonable artistic license, and it could be plausibly presented with stylized graphics as sonar range measurements.

Alternatively, the environment can be actually well-lit: a bat is only disadvantaged in daylight against enemies who fly better than he does and can catch him, or against enemies who can shoot him down, or against preys who can see it and run away, None of these categories needs to be a serious problem; many creatures remain too slow and grounded to put up a fight with a bat.

That might be true (don't know enough about bat biology, I guess bats would be really sleepy in bright daylight, but that might be just from habit of hunting in the dark).

Still, sounds like a wasted opportunity to not use some of the most defining properties of an animal in a game featuring said animal (or antropomorphic creature based on said animal, or superhero named after said animal) as hero....

On the other hand, if done too directly, could be to cliché, so what do I know.... maybe Mario would never have been such a hit if he would have had to fix pipes, instead of saving a princess from a monkey by jumping over barrels.

I really can't describe to you the terror of seeing a realistic looking bat butt stomping its way through a cartoon world. My kids would have nightmares. Heck, I would too. Let's just assume there is some cartoon bat style that doesn't look like Aero the Acrobat (who, incidentally, ALSO couldn't fly, so, heh).

This seems like a game that begs for multiplayer, and could actually be an incredible amount of fun, rushing around a large arena in a semi-co-op semi-competitive type play.