I think Sylvanas is one of the few factions leaders that received development over multiple expansions, she had her moments in Wotlk & Cata, played a minor role in MoP and is full force in Legion.
Sylvanas does not need more spotlight, as said above, Sylvanas being the figurehead leads to what i described, it becomes literally impossible to establish any other Forsaken character.
Aside from that, Sylvanas' position as Warchief remains whacky because her being Warchief makes the Horde no longer feel like the Horde, Sylvanas simply has a very different view on things than Orcs, Trolls and Tauren, the original Races of the New Horde.
Have you even played MoP?
Because if you did, you ignored the entire 5.1 questchain, which is probably one of the few highpoints Warcraft Lore had in the recent past.