As you finish each season it’s harder to get the tokens needed to make more pulls and as new hero’s come along old ones get forgotten.
It would be great to have some quest events where you can only use hero’s from specific families
i.e Atlantis quest where you can only use Atlantis hero’s in your team.
The rewards can be some Atlantis coins which would allow for more Atlantis pulls.
Hm… 
I get the feeling you have two ideas merged into one here.
The first idea is to address the lack of season specific coins to make pulls from the associated portal. Consider this as a possible solution:
The second idea is a quest/event idea of using heroes from a specific family/families to complete it. The issue I see with this particular idea is… what if the player does not have enough heroes from Atlantis to actually participate in it?