I think one of the big QOL issues right now is SWF and the lack of inherent AOE if you take that line of feats. There's only two solutions: either give it AOE, or make AOE less important in the game so single-target specialists are equally rewarded. I'm going to go down the first path because that seems more practical, though I'll caveat this by saying I think the latter path is more holistic overall.
I think we could hew a middle road between pure SWF and pure THF - and not just HaH, which still need the THF line, so its just "THF with an offhand" really (and way too much opportunity cost vs. actual THF). I'd accomplish this simply with a new feat, inspired by the work on Sunblades this patch:
Haft Grip
BAB 4
While Single Weapon Fighting with nothing in your off hand, your melee weapons will respect Strikethrough, and you gain +60% Strikethrough
Basicially the idea is you can wield a one-handed weapon with both hands and be able to swing through multiple mobs. Imagine a knight or a samurai with both hands on a longsword, swinging side to side with more power.
In terms of opportunity cost, its an extra feat AND giving up your offhand (except maybe runearms), which means no Guardbreaking, and no big Dodge or Doublestrike bonuses from named shield or the Shield Mastery line. That cost is more than pure THF or SWF and equal to HaH (assuming you need to use a feat for Bsword prof). In terms of balance, you'd have the benefit of 80% strikethrough along with SWF's 30% speed and 1.5x, which I think is in line with both THF (140% ST and 3x) and HaH THF (140% ST and 1.6x, with an offhand). You still wouldnt be able to qualify for most Strikethrough enhancements, either, since they usually require the THF feat.
I think that fills a niche nicely on the spectrum between pure SWF single-target and THF multi-target that right now isnt really filled by anything, and would breathe new life into a lot of SWF builds.