Few ideas I’d like to throw out there, some I’m sure have been floated around quite a bit, some are going to be controversial or just bad, but I thought I’d put them out there:
(1)
First, it would be awesome if an arrow key and an input key could be held down at the same time, this would make doing things like erasing a whole line a lot easier as you could simply hold right and ‘.’ at the same time.
(2)
Along with this the ability to select a large area, maybe using a modifier key and the arrows, to freeze/pause it and shift everything in the selection in any direction would be killer
EDIT, just realized you can already make selections by holding shift. This took me way too long to figure out ╮(─▽─)╭
EDIT Again: So, with copy paste, this is a semi-moot point, though still possibly worth considering, also it’s taking me a bit to figure some of this out. For example I repeatedly tried to delete with delete instead of backspace before figuring that out. Would you consider adding some documentation on github?
(3)
Next, as of now the grid can be zoomed in and out of as well as made bigger by expanding the window to a bigger size, but I think having the ability to scroll would be great. I’m not sure how to make this look visually appealing though. There’s also the issue of stray operators that may try to carry on indefinitely, so it may still be wise to keep everything finite 
(4)
Currently MIDI notes and MIDI CC are possible, but I think having MIDI Pitch and sustain would be nice too, Pitch, in part, because it’s so much higher resolution than normal MIDI notes and sustain would be nice for obvious reasons. I know operators are running short, so maybe MIDI Pitch could be done by putting two ‘:’ next to one another, and three for sustain, since it should only need two parameters (channel and state) ? just spit balling ideas here, I know this breaks the trend of one operator to one task, but your sorta running low on operators here ¯\(ツ)/¯
(5)
Finally, I’ve hit a few situations where it’d be nice to have a compact way to redirect a moving operator, say, take an E and output a N or a W and S, etc. I was thinking maybe using a modifer and an arrow key could insert Unicode arrows and work as these redirection operators. Obviously there are plenty of ways to do this right now anyway, but none that are as straight forward.
Edit:
Because there’s a lot here, I’ve numbered the suggestions to facilitate discussion