Favorite partners for Teletype:
16n Faderbank: adding more real-time control opens things up a bit. I especially like it for sequencing, or changing probabilities. I also frequently use Teletype to just pass the fader values over i2c to ER-301, but that doesn’t really feel like using Teletype.
Strangely, despite the i2c connection I don’t feel like TT and ER-301 are especially close otherwise; once I realized I had patched analog CV between them when I could have just sent an SC.CV 
The main way I use TT is a trigger stream manipulator – an algorithmic gate sequencer, divider/multiplier, etc. with occasional CV for modulation or sequencing, and sometimes using the CV input for S&H/shift register processing of another sequencer. Marbles fits into that very well and makes a good partner. To a lesser extent, other sequencers do too.