One choice is whether to make the game 'physics based'. That will determine what the player does to aim.
Some gameplay:
- one of the biggest issues in aiming at a still target is breathing. How to put that in gameplay? One idea: a moving wave where the player has to keep the mouse 'steady'.
- normal aiming at a static target. One idea in a non-physics design: have the computer move the gun a little and the player has to counter these moves to keep it 'aimed'.
- Find a way to do both of the above at the same time - say, one with each hand.
- Do you want just still targets, or moving like Skeet? Different gameplay.
- How about long distance 'sniper sim'?
Something like a police/military training sim where you go in a room and targets flash, do you shoot the bad guy or is it an innocent person?
It's not that easy to make such a simple activity all that 'fun'. Think 'bowling game' - but that has been done where people enjoy it, take a look at it.
Do you want a straightforward game, or one with things like good music and maybe some more animation, even 'fantasy' like talking targets?
For variety consider how to use different guns and have them be different playing, pros and cons. Of course, have things like 'achievments', stats.
Do you want the game to be 'educational'? Show the choices and making of bullets? The function of parts of guns? Commentary from gunmakers, olympic shooters?
Is this aimed at 'sim' players and serious shooters, or a general 'fun' audience? Kids?
Just like some chess games have hundreds of sets, how about different settings? Targets? Other players with voiced dialoge, like a poker sim?
I suspect it's not that easy to make a marketable marksman game, just like 'bowling'. I don't think I'd want to play one. So the heavy sim, educational, commentary parts seem like they might be good draws (ha, ha) for the game to the gun enthusiasts. I'd avoid things in poor taste like human targets, but that advice is what I think it right to do and might be against what could be a gameplay that would be right on target for good sales.