@Arkenne - Most of those are a pretty bad ideas on account of them being RNG. You do NOT want interrupt mechanics governed by RNG. Ever. That makes for entirely random matches with random balance. No. What you want is a controllable, skillful method of interrupting. Something that can be directly controlled by the player who's trying to interrupt the cast.

I'd actually like to see exactly what we have now, but with some minor tweaks:

1: Autoattacks no longer interrupt. Ever.

and...

2a: A player's melee attack will only interrupt a spell once per two GCDs. (4-5s, depending on Job and Skill Speed)

OR

2b: A caster can only be interrupted once every 2.5s, regardless of how many players are hitting them.

Naturally, things (read: healers) will need to be rebalanced in PvP (however slightly) to compensate, but at least casters would be able to cast again.