*unlurks to give his 2 cents <.<
You could integrate the slut/virgin theme into the storyline so that the plot changes depending how you play. Have a virgin playthrough for example with a different script and unique pickups where she only kills teh enemys and never screws them into submission. To this end you could also include a sort of levelling system whereby you spend points into various stats such as combat and sexual prowess. Make the stats multifunctional though so that virgin characters will still need to pump points into sex for a few obstacles. Ultimately make it so that there is almost always more than one way to skin the proverbial cat. That introduces some puzzle thinking for players that are role playing yet provides a streamlined playthrough for players who just want to blast through. A good story should be important either way though.
I'm thinking of Iji for most of those elements to be perfectly honest so have a look at that game if you haven't already seen it:
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The sprite looks good, I'm torn between the two versions. It depends on what you plan to do with the enviroment around the character. outlined is easier to see on a busy screen but non-outlined may feel more natural. Outlined makes her look thicker, suggestively more muscular, which may or may not be a bad thing depending on context. The definition of the details on the outlined version should be carried over to the non-outlined one tho as its hard to discern her jacket from her skin otherwise. Imo the face needs to be able to carry expressions though which may prompt a slight increase in size.
If you're worried about view distance relative to the characters size then have it so that the camera shifts depending on which direction the character is facing. I.E. if the character is facing right then have the camera shift so that she is on the left of the screen. Reduces the risk of having to make leaps of faith. Off teh top of my head I think Cave Story does that.
For the costumes either have them as armour to be torn off and have lots of costumes or use them purely for aesthetic reasons and have only a couple of unlockable. The former adds to the depth of gameplay a bit whilst the latter gives dedicated players a sense of achievement.
I agree on the gore front. Keep it to a bare minimum if at all. Ideally her defeats should at worst be only suggestive and can be taken one way or the other. At best I'd say a final rape scene based on which enemy claims victory over her.
As for vore thats a bit more of a grey issue. Vore where she gets devoured and raped within the critter should be fine. It should be okay where it is strongly implied she is not digested alive, rather kept alive at her hosts... pleasure.
Keep your enemies and rapes varied. Don't get too stuck on a theme. Male, female, androgenous. Barbarians, lamias, robots and rapeworms. Whatever. I'm not gonna run off a list of sexual interactions but I think you get the idea at this point: as long as it fits the story line without compromising variation you should be good to go. You could also introduce a random element to the encounters as well, say theres a chance the beasty will force it down her or let her cough it up or whatever.
For the difficulty of the game, you'd be saving yourself a large headache by introducing difficulty modes. Always let the players decide how hard they want to suffer!
Finally on character-enemy sexual interactions. Imo the "mash left and right buttons" thing has been done to death. That horse be glue. You could try some sort of more developed quick time event system where she needs to orgasm before the enemy so as not to lose any health or something like that, modulated by skills. Problem there is that you don't want to detract visually from the action but this is still supposed to be a game so I don't know how exactly you could reach a compromise between those two points.
Anyway thats some food for thot. And yeah I know some of its been said already. Just giving my opinions on those elements. Eagerly await further development on this one. ^_^
