The problem with Shield Swipe isn't that it's on the GCD or the potency, it's the pacification effect. Pacification is super overpowered on stuff it works on and useless on stuff it doesn't work on. Unfortunately, almost everything worth pacifying (bosses, mostly) is immune to it. This is a problem because we have Shield Bash and Blind also, which kind of have the same problem where they're really good effects but a lot of stuff is immune to them. Flash has other uses and Shield Bash has enough uses, so I'm not saying they need to be changed, but I think paladins have enough buttons that aren't useful during most boss fights that I'd like to see Shield Swipe changed.

I agree with the OP that it should have a different secondary effect. I think it should stay on the GCD, since paladins have enough off-GCD stuff already, but the effect should be something that's useful pretty much all the time, but isn't so powerful that you absolutely need to bring a paladin for it. It'd be stepping on bard's toes a little bit, but maybe something like a short-duration version of Foe Requiem would work. I've also seen the idea tossed around that it could heal players for a small amount every time they hit the target. It doesn't need to be huge, just anything big enough where you want to hit it when the button lights up most of the time. While I do use Shield Swipe now (it looks cool and makes me feel like I'm kind of doing things), the DPS increase is very small and if you want to regenerate TP then Flash is better hate and better TP regeneration.