Chip could go up to 8 MB without breaking much if anything. AGA implementation should be set to most compatible by default IMHO, with extensions hidden from view if not explicitly unlocked. I don't think a fast blitter is needed as the CPU is so fast you won't need it.
Heck, you could do whatever you want in pure software with that much speed. (Way more than a Neo Geo ever could)
Some high color chunky modes would be nice tho. (I'm not talking P96 here, but bare metal accessible modes)
We'll see how it'll turns out
Edit: what about Paula implementation, is that even on the roadmap?