I have three topics that I seriously collect:
1. Esperanto, Volapük, Ido, and other constructed international languages. I have stamps, postal stationery, covers, cinderellas, etc. This has been the major focus of my topical collecting for 25 years.
2. Runes and runic monuments. Mainly stamps and FDCs.
3. J.R.R. Tolkien. Stamps and FDCs. For stuff related specifically to the Peter Jackson films, I limit myself to New Zealand issues.
I also have a couple of Vario binders devoted to other random topics that interest me, but I don't try to be a completist on these.