I'd favor a more constant effect over random luck. With a WS paralyze, you'd probably still want to keep Stuns handy just to be safe, and they'd probably start casting as it started only for the stun to basically be wasted.

Mainly for increasing the ready time of TP moves (making stunning easier) while affecting their damage potential, be it some FTP manipulation or just straight damage reduction. Not sure how I'd feel about the pseudo-Convergence, since knowing SE, it might turn any AoE TP move into instant death for the single target.

As for additional effects on TP moves, there's probably some element of resisting and MACC in play, just that the mobs probably have stupid high MACC to guarantee they land most of the time. Addle may help here some, but a more potent, pure MACC debuff could do more alongside an appropriate barspell, carol(s), and gear. I'd just hate to see a point where tanks will need to start investing in resist sets for all 8 elements.

Assuming the actual spell list isn't doomed to some limit, maybe a Null-Slow and such could be added. Cast it on the target, and any time it uses that enfeeble in some capacity (Spell, WS, triggered by Aura), the "debuff" is eaten and the enfeeble doesn't happen, requiring you to cast it again to block the next one. I'd see something like this building resists akin to Grav and Stun, though, so it might be something saved for later in a mob's life with the ability to only keep one up at a time.