I'd buy some materials I've never tried, like Brovira paper just to see its qualities. I would never waste my money on materials like that now due to high prices these things often get up to and possibility of it being fogged, etc. HIE and EIR film comes to mind too. Stuff like that, nothing fancy, just more inquisitive materials to experiment and learn about. But I'm fine now not splurging on things like this...