What F!Morgan to build

If she's going to bait out mages/healers then +Res/-Spd is usable. On the other hand, if you're going to turn her into a BladeTome flier in a flier emblem then +Spd/-Res would work well.

Tbh, I'd go with +Res/-Spd due do her strong base kit and the advantage of have already high Res. I think Spd is a super bane? That can be covered with QR.

Depends how you want to play her.

If you're planning on going her Bladetome build, I'd suggest (+Spd, -Res) so she can take advantage and potentially buff her speed where she can double.

If you're going for a minimal investment/budget build, (+Res, -Spd) is a good way to make use of her base kit and turn her into a mage tank. I use this build and it's good for team support as well as baiting out mages and murdering them. Mine is (+Res, -HP).

She can be great asset to flyer team with owl tome as well if you dont have any units with hone flyer.
Here is mine (-atk +spd) works great.