indeed, just ignore that other poster - I’m certainly not going to judge your account just by what heroes you have nor am I going to be condescending towards someone asking for advice.
I would ignore Sartana - even with first costume, I don’t use mine anymore. she just does not do enough damage (compared to others I have). maybe this will change if you get her 2nd or her toon costume.
Domitia, without costume, can also be ignored - you already have Seshat to snipe and dispel.
that leaves Uthragan or Viscaro.
Viscaro has a good niche to punish minions, though is not the fastest. He might fit in well with your team - you rely on Wu and Cyprian, Viscaro helps add pressure on the enemy, punishes healers.
Uthragan could replace Clarissa, but not complement - their poison will overwrite, so you won’t have both their poison active at the same time.
(it would help with your depth, e.g. like in War, use Clarissa on one team, Uthragan on another)
some would say hold onto your Tabards; but before deciding that, it’s useful to know: how much do you summon? (that helps us answer: how likely are you to get a “better” hero, if you wait?)
you could also wait until the coming Costume Chamber: maybe you will be lucky and get the costumes for either Domitia or Sartana. (or even Obakan or Quintus, and you can work on one of those first while waiting )
overall, I think it should be either Viscaro, or wait.