Is it bad to build 
on supports like Karma and Morgana? I seem to start doing that if I've built my base items, and if the game lasts long enough I build 
. Is this, I don't know, OK? Or is there something I should definitely do instead?
GA is an AD item, so you should go with zhonya's (or frozen heart if you really need the armor).
I tend to just build resistances on support if I need to be tanky rather than health because things like Ruby sightstone and redemption give you a good amount of health.
if you're playing morg zhonyas is a better option then GA. on the warmogs, you can buy them if you are skirmishing frequently but there are better investments for teamfights.
