Lots of good suggestions in the thread. You have one pistol in rimfire and one centerfire revolver, so I would suggest a centerfire pistol next. Something in 9mm would be less expensive to feed if you shoot on a regular basis. It would also give you an idea of what you might like to investigate further.
Two additional suggestions: Never sell your first gun. Ever. If you do, you'll regret it. Secondly, after you acquire another gun or two, you may want to look into reloading, especially if you do a lot of shooting. You'll have to lay out more money up front, but you'll have an endless supply of ammo to feed your guns.
You sound like you're off to a good start. Enjoy!