Hi PsychNerd !
You can do that today already, using the "Midi Note Range" device source, and an exposed parameter. It's an experimental device so it's a bit hidden in the GUI.
Here's a quick tuto on how to do it:
1 - First, you need to create your MIDI device in the parameters
2 - Choose the thing you want to change through MIDI input.
Here, I chose the activation of different layers in the sprite point renderer of my particles. Right click on it > Expose as percentage.
I want to change the activation of the 3 different layers, so for the others I did: Right click the activation eye > Connect to > Dot White b.png Activation
3 - By selecting your parameter, you can edit it in the functions editor to activate the first layer, then the second and then the third
4 - Right click the value of the parameter > Learn
And learn any note you want
5 - Select the link and change the device link source to "Midi note range device"
6 - Activate HOLD and put the values of your notes (lower and highest).
Here I want the 8th note to show the first sprite, the 9th the second, and 10th the third.
By putting from 8 to 10 I tell it: 8 = 0%, 9=50% and 10 = 100%.
The exposed parameter matches that (sorry for the crudely drawn arrows ^^)
There you go !
That way, you can change pretty much everything, from the seed of a noise to the activation of objects, a color or event text.