Q 58a: Are there any creatures, in any 3.5 publication from WotC that have more HD than a great wyrm prismatic dragon (78)?
Um, you could advance that Dragon further, if that's what you want:
http://www.d20srd.org/srd/epic/monsters/dragonAdvanced.htmQ58b: You want abilities that scale with HD. Well, the DCs of (Su) abilities usually do... also, the phrenic template (and possibly other PLAs; I don't know the exact rules but I think they're augmented to ML; by default, ML=HD). Combine with magic in the blood for fun. Archmage can give you SLAs. There's also the feat Innate Spell (CA) that gives you an SLA of a slot 8 higher at will.
Uncapped spells I can think of are Wings of Flurry (though the dragon is too big to make use of it, I think), Maw of Chaos and Gate, though Gate has an XP cost that isn't easily done away with. You could possibly use Supernatural Transformation (or something similar sounding) from Savage Species... but then what would you call with Gate? Another Dragon buddy just like you?