Interesting, I am in the process of designing a 16 step 4 channel MIDI step sequencer. A device which can help you sequence in the way they used step sequencing in the 70's/80's Klaus Schulze style.
I am using a Raspberry Pi plus Arduino board and have a working very early prototype model with one channel and 4 steps, currently MIDI note on/off only. I am writing the software for the Raspberry Pi in Python 3.x, using one of Python's MIDI libraries. Currently I have started moving the software to a multiprocessing / tasking stage with message handling queueing so I can start focusing on tightening and better control the timing of the sequencer loop. The Arduino is handling the rotary encoder inputs at the front panel and communicates what is selected to the Raspberry Pi over USB (and provides power).
The idea is to create immediacy in the user interface: knobs, buttons and LEDs and a small LCD/OLED display. I really want to get away from the ubiquitous computer interfaces - just something that shows you immediately what is going on and gives you complete control.
From my perspective, obviously, the software will be Open Source / Free Software. Those who know me, who understand my passion for Free Open Source Software, won't expect anything else ;-). Perhaps in a year or so we will make a hardware product out of it, or some kind of self assembly kit, who knows.