Nothing jumps out as an absolute must 6* now type champ.  So it'll very much depend on your next area of focus. 

That said Shamael would be my suggestion just because he's so good in Hydra so he will always be useful.

i'd 6 star ronda, rector drath, ghrush, grizzled jarl & husk if it was me, but it depends what ur lacking.

I used Narma for a long time in Clan Boss, if you need help there strongly consider her.

Husk and Schamael are both great in Hydra. Husk has some utility in other areas as well.

Yannica can be powerful in arena as well as with the frost spider. (personally, I'd pick her over Ronda)

I'd strongly recommend *not* going with Narma. She's probably the single biggest regret I have in terms of using lego books. Very sub-par.

Where are you looking to improve? Are you done Faction Wars?

I know you say nothing is going to be a game changer for you, but would it be possible to see the rest of your account still? Does help in terms of complete information. I ask because some of the suggestions can end up being very fringe for some accounts, good for others and a bit of a waste on some. :)

Assuming you do Hydra and are looking to progress in it: Husk and Shamael. Those seem the two most guaranteed impactful choices to me.