if you want to get into hacking on a real ntsc/pal signal there is a whole universe of very expensive eurorack gear. on the low end you could check out ch/av for a VGA synth. you probably want something a bit higher level.

for a simple raspi-based device that lets you play with video feedback and has midi-tuneable features check out video_waaaves and other software by andrei jay. combined with a basic edirol video mixer and a couple sources this could get you pretty far.

for coding visuals, learn glsl on shadertoy, p5.js and threejs from streamers like yuri artyukh, and check out what people are doing with commercial software like touchdesigner or max/msp