I have a couple of suggestions. they don't need to be mutually exclusive ideas either:
1) a real thick, chonky tentacle can be used for single, or double penetration, especially for anal. I find the ones currently used are quite small for a hentai game.
2) someone mentioned before and I will do so again: some form of inflation;
a) inflated girls can move slower maybe have a different expression for being both satisfied and full;
b) inflated girls could leave a "trail" that might slow or otherwise stun (slip and fall in "it") other girls;
c) inflation could be gradual meaning you'll need to nab the same girl multiple times to get value out of it, or use a chonky boi upgrade for instant fill up;
d) along with the chonky tentacle, an option to "cum outside" and a bukkake effect in which different levels of being covered might affect the speed the affected girls move.
e) in addition to a bukkake effect, clothing that is too drenched becoming "transparent". I know it wouldn't actually be transparent but you get the idea.