Murkrow and Misdreavus are still only usable in Diamond & Peral (Which you should never-ever play with platinum out) so them. I'm pretty sure Dewgong isn't available at a decentt time in any of the games, so that Durant also would be cool and of course Wigglytuff. I'm not gonna say Farfetch'd though cuz that would pretty much gurantee it a new evolution.
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For the regional dex, I hope there's stuff that hasn't been in a regional dex since Emerald, like the Lotad, Surskit, Nincada and Gulpin families, plus Relicanth.
For non-native legendaries post-game (or whenever), I'd love to see the legendary birds, kami trio (non-roaming) and the creation trio (like how the weather trio were in HGSS for no real reason). I suspect Mewtwo will be in XY by virtue of its new forme(s).
Ditto and Smeargle should probably be in it somewhere too for convenience's sake.