I like Seriha's Regress the best. Although, I'd call it Discord a la FFVI. I'd make it potent and accurate with a very long recast and a very short duration, say 10 seconds. The point is that the whole party would have to pay attention to the window in which Discord was active so that anything that may be successful or super potent when the mob was 10 levels lower would be targeted for that time period. I'd like to see this potentially able to land enfeebles on the mob that wouldn't land before, and you'd have to have multiple people covering them because the RDM couldn't have time to cast all of them in the short window.

My point with this is to give RDM a spell that potentially makes everyone's attacks stronger and more accurate for a short period, one others would have to pay attention to, and one that we wouldn't be required (unable) to cycle due to recast. I agree we don't need more cycle spells. That's why I'd like to see some special case potent enfeebles like this.

To me Addle in some way was an attempt to address the most common complaint, that any remotely challenging mob is immune to any enfeeble that would matter. Of course any tough casting mob would be immune to Silence. So, introduce an enfeeble that is like 1/10 of silence and make it very accurate. Some people want more out of it but I actually like Addle.

In that respect I'd go with a Confuse/Inhibit to mess with TP usage and TP gain in some way that isn't as potent as, say, Amnesia, which would of course be resisted by every mob that mattered.