The thing is that Dolphure, Slowpriest and Sirene already play differently from one another. Dolphure has a speed advantage that greatly changes how it deals with other Pokemon, as well as Riptide. Slowpriest's arsenal of stat-boosting moves play very differently from Sirene's pure tanking.

And to be honest, the only way to get Fire-types out of NU/UU is to push them hard like Infernape and Heatran. In 4th gen, which Fire-types are seeing play besides the Ape and Heatran? The Ground/Rock/Water weak is just too devastating. Just because Physical Fires don't have an excellent STAB past Temper (which I would be fine pushing to 100 BP, since it really isn't quite worth the drawback yet) doesn't mean we can't diversify their movepools. It just has to be dabbling in different types. Most Physical Fires in 4th Gen use Fire Blast over Flare Blitz in any case, Flareon's trouble is an entire lack of good moves past Return, not just lack of STAB.

Arachmare is difficult to push because of its base stats, if I recall correctly all the movesets were done after the split, but were largely not designed competitively but more flavorfully by Ali. The idea wasn't to give each Pokemon what it needs but rather moves that fit.

Abyssus doesn't need any more pushing as is, Eclipse makes it possible Uber material already. Abyssus plays very uniquely (forming teams around his ability) which I believe is a good direction.

As for Hanzo, I would be fine moving him (Mistvale feels fine in Kirant as well) though Mistvale would need renaming.