I see no connection with Legendary mechanic and spears. That weapon could easily be governed by the archery skill. At most expand the existing tree to include a spear branch.
Its possible. Now they don't really have excuses for not added in new perks and perk trees.
I have seen this addressed several times. What does introducing Legendary have anything to do with... "No reason for more trees"? The level cap was set by the number of skills to start with. The actual level cap is, and has always been, hard coded in the thousands. If memory serves correct it is 26,000 or maybe even 66,000...doesn't matter. It's way up there.
At any time they could just add more skills and trees. Thus, increasing the level cap. Adding the Legendary mechanic sets nothing new in play, as far as being able to add trees. The trees governed the level cap, not the other way around.