I'm going to go out on a limb and try to answer things without knowing if they're reasonable yet...
I would love to see a visual indication of some Yukkuri personality traits. A gentle eyebrow for a nicehead, a harder one for a shithead, some yukkuri will be cuter, some dirtier, some with longer hair... mutations and multiple accessories notwithstanding. It would definitely be beneficial to split the body from the face for more dynamic types and to lessen the load on the artists.
I've always imagined crawling as the default method of movement, with bouncing as a kind of running and certain types being able to use that silly butt scooting for super speed. (Cheeeeen!) Rolling seems like a koyukkuri game (or Canrisa's awesome way of moving), and we'll definitely want stretching for shenanigans.
A full range of expressions would be ideal. These are human caricatures we're talking about, and they're famous for overreacting to everything without thinking much. The specifics will have to be worked out later.
Bleeding paste seems like a good idea. It certainly doesn't look like poo and it might just lead to cannibalism... To have any decent injuries we'll need cuts, bruises, scrapes, and we'll probably need multiple death animations. Jumping off a cliff is entirely different from dissolving in water. Having different injuries change how a yukkuri can move wouldn't be very hard to do. They would need to be done on a case by case basis though. We will need some death animations; even with the best of things, a koyukkuri is going to jump off a half meter table and kill itself every so often.
Eating is... hmm The descriptions that I have seen had oversized tongues, entire-face-in-food, bird feeding, super inhaling, and spoon or bottle feeding. The only ways I've seen drinking are like
this or
this. My preference is for face stuffing with neat and messy style, entire-face-in-food for madness, and somewhat oversized tongues for drinking.
It seems that love children lead to mammalian births, while lust children lead to stalk births. Both rub-rub and penipeni mamumamu should be in the game, as well as the base instinct of poo poo, pee pee, rub rub, lick lick, puff puff...