I'm sure if everyone here is joking with you, but you don't need a new stem. 1 1/8" threaded stems do exist, but they're pretty rare. You only need a new stem if you're unhappy with the handlebar positions offered by your current one. If you really want to have a bunch of stems on hand to swap out depending on your mood, your best bet is to get a new fork, headset (and stem!) to switch to 1 1/8" threadless. Personally I don't think this is terribly necessary. Once you find a stem/bar combination you're happy there's not much reason to change it.

If you're looking to upgrade, buy the stuff you'll appreciate the most. If your saddle is uncomfortable, get a different one. If you're not digging the drop bars, get different ones, etc. Do you ride with clipless pedals? If not, that's something to try. They're definitely worth it.