read a lot of newbie posts - there are tons here... it'd probably have been a good idea to have done that before actually buying anything - you will run out of space before you know it with such a small case
I'd look at mixing and modulation, at least as important as anything else you could possibly add - probably an fx aid would work well - has a lot possibilities - if you get the xl version you get cv-able bit crushing on all algorithms
see my signature for hints on how to get the most versatility for the least cash...
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities