I'd like to see Chainspell implemented as a cooldown to be honest. I'd like it to make your next six spells have no cast time and a recast time of 1.5s, matching the melee combo. Give it a 2 minute cooldown. It would may need a damage penalty, since it's roughly a 60% boost to your DPS for 9s with free movement while being a massive white/black mana generator on top of that because it applies to all spells, including Veraero/thunder. Combined with Manafication and a single Dualcast, you can get from 0/0 to get your melee combo rolling roughly 12s into the fight. The result of this is you get to do your full melee combo with Verflare/Holy during a standard 5th GCD TA opener. THAT would be a useful ability to give RDM.
I would never want to see it on BLM. Double-Triplecast with super leylines.
The MP nosedive would also suck tbh.