I think that's it for birds for a while, one of the Gen 5 genies has a bird form, Yveltal is more of a dragon, so yeah, no more birds for now.
The Lake Trio are all basically Diet-Mew, so making them cats made sense, I do wonder what they get up to in that ball together *wink-wink*
I'll definitely be trying to capture the nightmare vibes for Eternatus, especially when she goes into Enternamax form.
Regigigas was always going to be bulky and yes, Palkia will be too, Giritina I'm thinking chubby and bottom heavy but not exactly fat, Dialga will likely be slimmer still, Heatran is exactly what you guessed!
4.2 is exactly as far as 4.1 is... technically, there is a scene in 4.1 that can't be accessed yet, so that could be considered part of 4.2...
But I won't be working on PokeBreeder for a couple of weeks, so that's as far as it goes for the moment