It depends which ex trials and 24 mans really. Copied factory for example is far too easy whereas Sephirot ex would be hell.
With regards to actual dungeons, at the very least a general difficulty increase to similar to the original pharos sirius would be a good start. It wasn't hard by any means, but lots of dungeon players don't even bother to dodge aoes.
In terms of where they would be in the gearing. Well there's always a gap in the levelling process at least for armour and accessories. For example: there's no ilvl 465 armour or accessories nor ilvl 515. Accessories are also missing at ilvl 495. There are extremes at these levels that award weapons. 465- hades, 495-wol, 515-emerald
Normal story dungeons are always catch up in terms of gear level.
I would say the difficulty should be the likes of tower at paradigms breach at a minimum and Titan extreme as a maximum. There's duties easier and harder on both sides for both difficulties.
Certainly you don't want them as undertuned as the majority of shb's extremes have been. (17 deaths vs varis and we still cleared with time to spare? ridiculous)