This is an area I still struggle with. Although my first priority in collecting is, and always will be, Star Wars...I do find myself interested in a number of other lines. Even though Star Wars alone is more than enough to keep the checking account drained and our office crammed, it is sometimes fun to have another little "secondary" line to keep up on as well from time to time. I should probably just stick to Star Wars, and might still do that, but if I could somehow limit it to just one other line...and not completist of course in that line...it might be easier to do.
Also, "collecting" another line has different definitions to the Mrs. and myself. For example, there are about 3 more Lord of the Rings figures I'd like to get (having 10 total), and then I would be satisfied with that collection for now and not buy any further. To me, once that is done, I am no longer "collecting" that line. My wife feels the opposite, that if I have anything from a line, I'm collecting it. Which sometimes makes it more difficult to pick up the odd figure here and there, although she does support that to some extent...some of the time
That said, going by my definition, I'm only actively collecting Star Wars right now. I do however still check out other lines, namely Lord of the Rings, Marvel Legends/Spidey, and Batman/Superman stuff from DC. From the sounds of things, but Batman (comic) line isn't exactly going to be taking off anytime soon (and might be dwindling down), so that might be out for now. I have bought a couple of "the Batman" animated figures, but I'm not sure if I will continue with that or not. Lord of the Rings, like I said, is almost complete for me...still needing a Frodo, Aragorn, and Sam. That line would be fun to go further with, but I could be satisfied with my collection after those three. With the lack of a good Batman and/or Superman line out there right now (outside of the more expensive DC Direct stuff), I guess that leaves Marvel Legends. I had somewhat gotten away from them for awhile, with nothing new here all year, but then seeing some of Wave VI yesterday, it brought back the fun memories. Aside from the currently ridiculous case packing problems, it is a fun line to collect.
When it comes down to it, I might just end up sticking to Star Wars only...or Star Wars and the odd figure here and there, although it would be nice to be able to limit it to one other line, if any at all. It is probably not that big of a deal, but just something I think about from time to time. Do you think you could do it? Limit yourself to Star Wars and one other thing? If so, at this point, which line would it be? Just curious how some of you feel about it at this stage of the game, knowing what we know about various other lines.