I have been toying with the idea of creating custom midi effects (eg. arpegiators etc) for Ableton but I can't seem to find any info on how or whether it's even possible. All information about writing plugins seems to be focused on the VST side of things.
Is it possible to write custom midi effects for Ableton and are there any good resources/api references?
Yes, you can write processing in Max for Live. Unfortunately, that's about all I know about how to go about doing that, but it appears to be a GUI-based development environment, so perhaps it's not too challenging.
The Cycling 74 web site has free documentation, including tutorials, and set of MIDI tutorials.
There are actually quite a few programs that can connect to Ableton that don't require you to understand the vst plugin space. The most popular of which is Pure Data, the open source predecessor to Max/MSP. Ableton has a built in interfacing with Max called Max for live but since you have to pay for it I suggest you get accustomed with the interface using pure data which can be used in Ableton as well as other DAWS. Or you can do the 30 free trial of Max/MSP. If you would like something more closely related to programming and would be interested in live coding you can look into Supercollider and SonicPi. They both have good documentation and many people provide YouTube tutorials for a variety of tasks. If you're just looking for a more modular approach to software instruments and affects then something like Reaktor should suffice. I hope I've at least sent you in the right direction. Here are some links to the official documentation and/or tutorial pages for each suggestion.
Reaktor Pure Data Supercollider SonicPi Max/Msp Max for live
