I like the official merch but my favourite gaming/film merch is the subtle stuff made by people like https://www.insertcoinclothing.com/ (for games) and https://www.lastexittonowhere.com/ (for films).
The idea is, instead of directly featuring the game or film’s name you use something more subtle like a logo from an in-world corporation, or bar, or something like that.
The fun thing is, for someone who doesn’t know the game, it’s just a logo. But when you pass someone who is into the game they will go nuts (I have been approached several times about my A.G. Systems t-shirt!)
The Insert Coin stuff, especially, is really good quality.
I would LOVE to be able to buy an Ether Biotech or Sanguine t-shirt! Anyone else?
Also, these are officially licensed so some of the money would get back to IOI.