Well, personally I love collecting any species of plants I find attractive. I have a small, unimpressive collection of tillandsia species (air plants) which I have no plans to expand.
Then, there's my new collection of orchids. In addition to the usual dendrobiums and vandas, I've added a couple of unique species, including brassia rex, gongora claviodora and bulbophyllum lepidum. I hope to get more additions to my modest collection in the near future.
But my main and largest collection of plants are carnivores, which I have started collecting since 4 years ago. It's been my family's pride and joy, and they enjoy showing my collection to guests. Unfortunately, my carnivores suffered a great decline during my neglectful and busy period throughout last year. Now that I have my own section of my house allocated for my gardening activities, and my more efficient schedule, I'm restarting this collection. Hopefully it'll go back to it's former glory.
So, what collections do all of you have or hope to have?