I'd prefer if they didn't do that, tbh. The characters all ready have established roles (Maleficent, for example, is definitely a villain, and not really sympathetic at all like she is in the Maleficent movie... even if she is a bit of a joke in KH now), and adding the live action versions of themselves would just complicate and change things that probably shouldn't be meddled with at this point. They'd probably have to give them new character designs, too.
The ONLY way I can see this working is if it's Alice in Tim Burton's live action move of that (which is a sequel, anyway). So KH could just explain it as another trip to Wonderland when Alice is older (and then rightfully update her design).
And MAYBE Once Upon a Time (though I really hope they don't try this, either, because it could be a disaster), seeing as how the events in OUaT happen after the events of their respected movies.
Like, for Snow they could just explain that the Evil Queen survived the fall, after all, and then used her magic to change their entire world into Storybrooke. They could then deal with the first season, or maybe the second half of season two that deals with OC villains, but they'd have to make sure not to include any of the characters that all ready exist somewhere else in KH's canon, like Belle for instance.
And as for Aurora, they could find her in her world (but OUaT's "Enchanted Forest" version) right after Phillip gets his soul stolen by that monster, or whatever happened to him (it's been a while since I saw that last), and then detail her journey to try and save him (obviously leaving out the bits with Mulan).
But really, I don't want them to mess with OUaT at all (only if they were really, REALLY desperate, and needed some new plot lines for returning worlds, like in KHXV or something... though I really hope the franchise doesn't go NEARLY that long). So I just really want it to be Tim Burton's Alice, or nothing, but this is all just my opinion, of course^^ You're more than welcome to disagree.