Why I think they only have 5+ ultimate is because for console controllers that's really all there's room for.
But what I was thinking of is at level 15 they unlock the backup ability bar, maybe at like level 25/30 they could add a 3rd ability bar which could just be for buffs or an option to have the 3-4 main abilitys you use on the main bar and room for the other skills you have to interchange or such.
On consoles it's just a tap on the left direction of the d-pad, maybe for the 3rd bar it could be tap on the right direction d-pad.
Just an idea, open to discussion
Thanks for reading/comments