I've always had a bone to pick with the prestiges in DDO; rather their implimentation. There are plenty of prestiges that in tabletop 3.5e and even other 3.5e games (Neverwinter Nights, Neverwinter Nights 2) that are either multiclass based or are available to multiple classes.
Now that we've seen two uses of the universal enhancement trees, I'm hoping that the devs could unlock these 'multiclassed' trees to universally appear for the appropriate classes if requirements are met per tabletop stylization.
For instance, the first that comes to mind is Eldritch Knight. This tree is literally a prestige for Arcanes that essentially gives them a splash of fighter levels, without taking fighter levels. Why not allow fighters to take Eldritch Knight? Have them take Magical Training as a feat. ( https://dnd-wiki.org/wiki/SRD:Eldritch_Knight )
Likewise Arcane Archer required a few feats and the ability to cast level 1 arcane spells. Give wizard, sorcerer, and warlock access to these: with the inclusion that they require Point Blank Shot, Precision, and Weapon Focus Ranged. ( https://dnd-wiki.org/wiki/SRD:Arcane_Archer )
Palemaster should be available for Clerics and Favored Souls on the requirements of: Blood of Vol religion and/or Death Domain for cleric. This is based on the tabletop requirement of 8 knowledge religion and being able to cast both command undead and vampiric touch (which doesn't exist in DDO). ( http://dnd.arkalseif.info/classes/pa...ter/index.html had to dig this one up on another website.)
Seeing as Swashbuckler was always a class on it's own, though it shares similar ground with both fighters and rogues I'd suggest perhaps opening the prestige up to them. (Ranger as well if we consider Eberron official artwork of a certain Aasimar noble...)
You could eventually get even more crazy and make arguments for certain enhancements being opened to clerics based on their domain selection, but that might be a bit too far. Honestly, I'd make my argument for the first three and let the community make a decision from there on out with a dialogue with the devs.