I don't think the moviegoing public in the USA will embrace animation as an adult medium to the degree that would be required to make a Pixar-level production financially viable. Yeah, some adults like Japanese anime and some like Bashki-style stuff, and some (like me) enjoy going to Pixar movies even without kids. But all of those people are a minority. Overwhelmingly, "cartoons are for kids".
Besides, it would be an extremely stupid business move for Pixar. They've done one of the hardest things a company can do...they've made their name a brand synonymous with a specific kind of movie that people can trust. Parents can see that there is a new Pixar movie and they immediately know they can (and should) take their kids to it. Not only that, but the quality of their films is consistently high and far exceeds the hit-to-miss ratio of any other production company. What you're suggesting is akin to telling Coke to stop making so much cola and maybe make some iced tea instead, simply because you're bored of cola. Why? People want so much cola that they can't make it fast enough.
The only way this could possibly work is for Pixar to spin off its own adult-themed distribution arm like Disney did in creating Touchstone, and to slash the budgets. But even then I think it would be a colossal disaster...lower budgets would reduce the quality of the films and tarnish the Pixar name, and the well-established cultural idea of cartoons being for kids would make this a niche market at best.
Oh, what a surprise, Ivy thinks it's a bad idea. Do I sense a personal grudge?
If you take Beowulf as an example. It is a very good arguement that adult hollywood style animations is not going to be the norm any time soon. With production costs at 150 mil and a gross of like 90 or so, they lost out on quite a bit with making that movie.
Of course something could probably be said about the high production costs. Was that really neccesary? A think a good production team could make a similar quality animation for much, much less. Then it might be feasable.