a really useful type of module if you are a guitar player is an expression pedal interface...
doepfer and addac both do them (iirc they both do a variety - some can cope with on/off pedals too)
the addac one may or may not be diy, can't remember, but there is also the 7 dials cv express, which is DIY, if that interests you
being able to control at least some of the module whilst playing is quite useful...
"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