What to add depends on what you like to do.

My IIfx have PC cards, DSP cards, ethernet, Jackhammer SCSI, framegrabbers, sound cards, etc. You can also do multiple monitors if you like that sort of thing. What a IIfx buys you compared to a IIci is all the empty nubus slots plus a super fast 030 CPU. For simple use a decent video card and networking card are enough.