If Casual Content is Rouls Alliance Raids and Island Sacutaries, and Hard Core Content is Early Savage clears Savage Critireon and Ultimates, Midcore is everything else. Let me explain.

Midcore, is when you do a bit of everything, you raid but parsing and clearing week 3 isn;t your goal, clearing in 2-3 months is fine and if you get some blues and purples, cool. You like rouls and the side content is p neat, so you don't skip cutscene, and don't mind getting Crystal Tower a lot. You'll possibly do an ult if ur group feels up to it, you do hunts and grind old relics, thats midcore currently.

I think what people are looking for is more gear progression, dungeons not being so braindead and Alliance raids having the difficulty they used to have back. Critireon is a great content type that can easily fit into what Midcore is, and if it offered rewards and they affected your gear prog along with EX and Savage and tome grinding, it'd be dope.