A full wish list would take forever (I'm 
terrible at making decisions), but if you're 
talking about next-to-impossible, here goes:

Japanese animation =

Candy Candy: Anthony

Clamp In Wonderland: Clamp Campus Detectives 
rescuing Magic Knights

Eretzvaju: cel from game opening or impossible-
to-find OVA (looking for the OVA as well)

Fancy La La: La La in concert dress
(ending sequence or first stage performance)

Jin-Roh: Fuse (particularly in armor)

Macross TV/DYRL: Max Jenius

Macross Plus: VF-19 or VF-21 in robot mode

Memories: Magnetic Rose episode

Nadesico: Yamada Jiro

On Your Mark: any (have seen 3 in my lifetime)

Whisper Of The Heart: The Baron
(have seen only one cel so far)

Ys I & II: anything decent


(I never really actively searched for American 
cartoon art, since the prices are generally out 
of my reach.  So this list may not be so 
impossible...?)

Western animation =

Flight Of Dragons: Melisond (or anything else)

GI Joe: Duke, Hawk, Quick Kick, Snake Eyes

Silver Hawks

Transformers: Jet-Fire, Optimus w/Matrix, death 
(or ressurection) of Optimus, Dinobots

Voltron: Voltron forming blazing sword, entire 
team yelling "Go Voltron Force"

Okay, I think that's enough day-dreaming...