Considering coming back for Legion

[Baltyre]

Hi.
I made a break from boxing (i played all the MoP xpack, go into FFXIV, and get back few weeks on WoD), and i'm considering coming back for Legion.
Last xpack i give a try but i was not really satisfated, world seems empty without any players outside, it give me the impression that everyone afk on garnison.
I like lively world filled with players.

I read about the world quest system that remplace dailies, sounds interesting, but as a group, we will be only able to complete group quest ?
Solo quest are doable on groups ?

I wondering about the size of my team too.
Back on MoP, heroic dungeons were not really rewarding and i was considering shrink my team size from 5 to 3 (easier to integrate LFG raid for exemple).
I'm wondering pro/con for a 5 man or a 3 man team on Legion.

PS : Sorry if it's not written well, english is not my main langage.

PPS : Bonus question : My accounts are filled with charaters, did they increase the maximum number of char per world for Legion to let us play an extra Demon Hunter ? Or i will have to delete a char :/