Noone seems to have mentioned this, but if its a prequel, will we end up being unable to import and being only able to export to bg1?

I'm 99.9% certain you won't be able to import anything into BG3 (and it's even less likely that you would be able to export into older games). If BioWare didn't bother with importing to NWN, importing to BG3 which will use 3.5e rules (and a new engine, developed by someone else) is even less likely. Especially if it really turns out to be a prequel, characters will start at level 1 again anyway.

Characters starting at lvl 1 is just gives away the most obvious answer to this question... The other not so obvious answer is the new DND rules. But Taluntian already mentioned that...

it's obvious that importing to bg 1 will be impossible; you start bg1 at lvl 1, so that would
leave very little room for improvement in bg3.... :D

Well that makes sense, how would they be able to explain the story when at the end of BG2 you either become a god or stay mortal?