I think I've settled on FB Deadeye as league starter. It can easily fit the standard MF template, 2 inspired learning jewels, and all the gear is cheap. I was originally going to league start with an arc trapper but I was having trouble fitting 2 inspired learning jewels and the typical trap passives, even after dropping crit passives.
Also, as Kimble pointed out to me, traps don't proc Heralds.
After that, most of the MF gear will transition to a poet's pen build. And not sure from there. I'm planning on going Hierophant ES Indigon but an excellent point was made during Baeclast that this type of build will likely be one of the best for delves, thus making Indigon ridiculously expensive. Probably making (good) Watcher's Eyes even more expensive.
Might have to wait and see what my "finish up challenges" build will be.