Part of it is legacy-thinking, I believe. Folks are used to spellcasting characters having pretty large repertoires of spells in various editions of D&D, so it seems like the selection in DCC is very small. The fact that DCC spellcasters have far, far fewer spells than their D&D counterparts makes it a non-issue in reality, but until that realization is fully grounded, the urge for more spells is going to be a natural one. Plus, as gamers we simply love MORE STUFF (tm).
I think you're on to something here. When folks come up with a "wish list" the natural default is to think: "now, what have I always needed in other games?"
The quick answer is usually monsters, spells, and things like that. But really we won't know until we have a chance to play it more. I've been playing DCC for more than a year now and still haven't really used up everything in the rulebook. Most of my players have tried a few but not all of the class options, and we haven't gotten characters to the tippy-top levels yet to exhaust the choices offered there. I suspect I could play for another couple of years on the core rulebook without many other extras.
I guess the things that I find most useful:(1) Modules
-- but of course this is happening already. (2) Setting choices
-- I wish we could get Blackmoor, the Wilderlands, or Greyhawk officially released in DCC format. Or maybe a literary setting from Appendix N like Nehwon
(Fafhrd & Grey Mouser), the Young Kingdoms
(Elric), or Hyboria
(Conan). I'm looking forward to some of the settings out there, but would love to have something *BIG* that really attracts gamers and says "this is why you should play DCC. You get to play in __ setting!"(3) Character class options
-- I still have folks who want to be Rangers or Assassins or Paladins.