TLDR: Huge damage, induction-based xbow attacks in Fighter Kensei.
I think, of all the weapon types in game, non-repeating xbows are the most in need of improvement, and the most ignored (even moreso than throwers). AA skills dont work with them, Bow feats dont work with them - they require their own unique feats, their damage isnt markedly better than bows or even throwers (which are also Simple weapons), and repeaters are universally better and auto-granted to Artis and Mechanics. They're a weapon class in desperate need of a role.
I think a role that currently isnt filled, that xbows might excel at, is a 'siege tank' gameplay style. High physical defense, coupled with high-damage, slow-rate attacks. Especially since Great Xbows have high damage and a good crit profile, but still not enough to match the DPS of a repeater or bow with Manyshot/10KS. Adding in some new xbow-only skills might help to flesh out the role of a dedicated xbow wielder...the same might be extended to other underutilized Kenseis (maces and clubs, picks and hammers) but I'll just focus on xbows for now.
I'd link this to Fighter Kensei as the most logical place - fighters dont need light armor, have the bonus feats to support all the xbow specific feats, and are designed for physical damage (both giving and taking).
I envision a few unique skills, autogranted through Crossbow Kensai (Required: equipped Light Xbow, Heavy Xbow, Great Xbow), that are induction-based (so you have to stand still while firing; cooldown not initiated until induction complete):
Kensei 1, Arm Shot: 1s induction, 3s cooldown. +100+[FGT lvl*2.5](max 150)% damage, target takes -4% to-hit debuff for 10s, stacks 5 times.
Kensei 2, Flesh Wound: 1s induction, 6s cooldown. +100+[FGT lvl*5](max 200)% damage, sticks a bleed of [FGT lvl/2]d6/2 secs, over 12 secs, stacks to 5, loses 1 stack on completion.
Kensei 3, Suppressing Fire: 2s induction, 8s cooldown. +200+[FGT lvl*10](max 400)% damage, deals damage within a small AOE centered around your target (about one medium monster width in radius)
Kensei 4, Hand Cannon: 3s induction, 6s cooldown. +300+[FGT lvl*10](max 500)% damage, +1 Crit Range and Threat, GXB knockdown on crit instead of vorp.
They'd have to introduce a couple worthwhile named LXB/HXB/GXB alongside this suggestion, but I think it would make for an entirely new niche of ranged physical combat that wasnt centered around Ranger splashes or Repeater Artis.
The idea is that you can be a heavy-armored, high-PRR and AC-focused ranged combatant (with the other Fighter tree enhancements), that can stand there delivering huge damage at range or point blank, that makes up for your sacrifice of mobility, and also able to tank. IPS would let you multiply your induction-based attacks even more, but that would require a lot of player skill to keep mobs lined up right while you induct.
The tradeoffs are being FGT lvl-dependent, so you cant just splash FGT 4 and then go Druid or Cleric or FVS for awesome self-heals, etc. Also, you're using Xbows, which have no stat-to-dmg/PA, so most of your damage is coming from FGT levels increasing your mult. Lastly, you're sacrificing 1-3 seconds of inaction while inducting, which averages out the huge multipliers.