Simple: give them more fragment slots in their aspects and undo prior nerfs to them. It's really that easy until they get the complete rework they desperately need.
Things like Knockout, Bastion, Offensive Bulkwark, Juggernaut, and other two-fragment Titan aspects should have a 3rd fragment slot. Giving Titans the ability to get an extra +10 stat points in PvP (because let's be honest, that's all people use them for) or the ability to make a slightly better PvE build will not break the game, or even substantially move the needle on their power. It will be a nice bump in usability without altering too much at once.
And things like Knockout should have their prior nerfs undone. I don't see why this is such a problem for Bungie to undo nerfs to revert them to their past states and admit they made a mistake. We already know how powerful they were in the past because that used to be the status quo, so it's not like it'd be an unknown factor.
Remove the dumb, major HP nerf on Knockout kills that it got at the start of TFS. Make it count as a powered melee again, because it [i]is[/i] a powered melee, and the so-called "punch class" should be good at punching. Letting it generate a single orb of power on a melee kill isn't gamebreaking lol. If things like Threaded Specter get three fragment slots, then Knockout and other two-fragment aspects absolutely should, especially when you're practically forced into it because the other aspects frankly suck.
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Rebuffing only comes to fruition for the Hunter classes, exotics or abilities.