I got back into the hobby way to late after initially having fish when I was a young pup. If i'd have kept at it years ago my list of "to do fish" wouldn't be quite as long because i'd have already kept them, had the experience of them, and moved on to the next on the list. However, back then it was what was going on in my life that determined whether I had tanks or not.
The fish i'd like to keep before it's too late would be frontosa (the amazing crazy blue variety), datnoids, maybe a vibrant, crazy colourful African cichlid set up and number one on my list, the one i'm actually likely to do first is a cold water set up with a shoal of european perch, the first fish I ever caught as a lad and fell in love with. I think it would be fitting that the last tank I ever have will be for the first fish I ever caught.