Cast adult voice actors for the characters: who would you cast?

There's little question about it that the first film is awesome and the sequels get progressively worse, culminating in the TV show, but I had an idea: what if they made a reboot (well, not really a reboot so much as a final definitive good film to end the series on) which featured the cast as adults? Who would you cast as the voices?

Tone: darker (but not necessarily more violent) and more mature - think "Legend of Korra" as a point of reference. It's still an adventure cartoon for kids, but the characters have grown up and are tackling adult problems; the story has no singalong songs, uses a darker and less vibrant color palette, tackles themes like death and destruction, causes the heroes to actually undergo great hardship and struggle to earn their happy ending, and of course has a genuinely scary villain.

Cast:
Adult Littlefoot (Bigfoot?): Nathan Fillion
Adult Cera: Jennifer Hale
Adult Petrie: Steve Blum
Adult Ducky: Tricia Helfer
Adult Spike: Dee Bradley Baker
Adult Chomper: Benedict Cumberbatch (Smaug)

What do you think? Who would you cast?