a mixer and an I/O module to go external though some of our members here do this without the I/O module, up to you, can't say haven't warned you to be careful then with DC signals ;-) I haven't even mentioned other utility modules yet, which would be the next step and only after that the more fancy looking modules (if at all).
@GarfieldModular, what would you recommend for an I/O module and other utility modules?