Just discovered this game, it's great!
I love the addition of the poses. I agree with the poster above who suggested lotion to massage the butt, I think oils could be good too. Maybe squirt some oil on and see the panties become translucent, have it drip between the crack and out the bottom of the panties. It can splatter around as you smack the booty.
Skirts would be great, and wedgies could be a lot of fun. Make the lips red as you pull tighter.
I know you don't want to turn it too much into a sex game, but a little more endgame content could be fun. I think when you grab in the middle and spread out, (especially if there's a two-hand mode) having the lips/butthole open up a little would be a nice touch and look more natural. Who doesn't love a good spread?