Luke is writing Sera. People do seem to think Weekes is writing the Iron Bull, (slightly OT to this OT, will he still be writing the majority of the TME characters do you think?). Gaider has Cassandra and a mysterious other. Kirby had Varric and Vivienne.
Edit
And Sheryl definitely DOESN'T have Cullen.
I would imagine that Patrick is definitely writing TME characters, since he'd probably have an easier time finding their voice. Same with David and the Asunder characters.
1. I like to think Weekes would handle any TME characters we encounter, but I'm not sure how they divvie things up in the writers pit. I seem to recall that for ME3 different writers handled different specific plot lines and that is how minor characters were handled, with input from leads/help from others. But I could be wrong or the DA team may well handle it differently, who knows.
2. I can definitely see Weekes writing IB and, if so, I'm pretty sure I'll love the companion... Even if I wish he was a rogue, still

You know, I asked this question only to realize I don't even know the full writing team and who all has written whom previously. Whoops.
Anyway, I have to admit, I'm kind of hoping someone "new" (as in hasn't-written-Cullen-in-the-past) takes on Cullen, if only for a fresh take.