I'd be okay with the capture/release aspect of a Runic ability that did take a party member's spell, preserving all its statistical info to be released by the RDM at their discretion. So say early on in a fight you absorb a BLM's Blizzaga III, later on a darkness SC is set up with the BLM bursting the same spell and you then release the one you'd been holding. That could be a potential instant 10k if the BLM's pimp enough and a sizable chunk of any mob's HP.

Beyond that, this is kind of creeping up there on my list of disdain like the old and tired Double Cast idea. I just can't see SE giving an ability to completely negate a any mob's spell (Utsu at least fails to -agas or SCH-buffed STs), let alone at a reasonable timer. I'm quite bluntly sick of any JA having a timer greater than 3m these days, as it only perpetuates FFXI's slow combat compared to other games and feeds into the lack of diversification among jobs. Anything too short, however, and you risk multiple RDMs in an alliance basically shutting down a caster mob, which are usually defensively squishy and only a threat because of said spells.