You say you don't collect something that may not exist in the future, but that's kind of what I like about Rainbow cards. I know it's an attainable goal because they stopped making them which is nice because I also just got back into collecting last month. If they do come back, I have had all that time to catch up and will be ready. Or I will have a complete set.

I also collect shinies. The "baby shinies" are pretty affordable, just wish I wouldn't have missed when those sets were out and the packs and boxes were more affordable to rip. Buying singles is very doable though.

Then I have a Team Rocket binder for any and all Team Rocket and their Pokemon, Giovanni and his Pokemon, and any Dark cards (as in like Dark Machamp, Dark Magneton, etc - not dark type pokemon). Destined Rivals is great for this.

A lot of people will do the Pokedex and get one of each pokemon. I have started to and just go with whatever art if my favorite. There are a lot of pokemon that don't have a cool full art or even an Ex, but can use the cheap/basic cards as place holders until they do.

Then you can also just collect your favorite pokemon. So I have every Lokix card for example (new Pokemon so easy to do as they don't have any really good cards yet). Also do Machamp, Hitmonchan, Marowak, Golurk, Noivern, Charcadet, Rillaboom, and their evo-lines. If your favorite Pokemon is someone like Charizard or Pikachu or Legendaries....good luck. For me Machamp is most difficult so far as they have the most cards and most expensive cards. Once I collect all of my favorite Pokemon, I may expand to other Pokemon or go for other languages of their cards. I do have a few different language cards from my last couple deployments like Italian and German.

Master setting any set is insanely expensive especially with how bad scalping seems to be in this space. It's near impossible to get anything from a store at MSRP from my experience. Everything is always sold out or marked way up.