Hmmm.. tough call.  I do sometimes ponder the notion of 
getting out of cel collecting, and think about that issue 
of would I keep anything.  
I guess the cels I'd keep come in two different categories: for the cel itself, and sentimental reasons.  First on my 
list of stuff I'd never let go of is actually my signed Ifurita cel.  Not so much because I like the 
character/image (though it is lovely) but because I enjoyed 
talking with the character designer so much I couldn't bear
 to part with the cel he signed.  
And I guess the other is my Larva opening cel.  While I 
must say there are quite few cels I own I never thought I'd 
get my hands on, that is the one I wanted most and was most
 certain I'd never lay hands on.  A close second is my new 
Hotohori cel, just because it is a testament to keeping a 
positive attitude, since I'd been telling myself since I 
started collecting "don't worry, you'll get one someday".