Fortunately, I've reached the point where I've acquired just about all the guns I've ever wanted and I'm now just adding unique or special firearms.
IMHO, I believe every "collection" should have...
... a 22 pistol - you can't go wrong with a Ruger Mark I/II/III
... a 38/357 revolver - Ruger GP100 or S&W 66/etc.
... a 1911 - lots of choices but I'd save up and buy a good used pistol rather than a cheaper new one (I like my Colt Commander and Kimber Custom II but there's lots of other good choices)
... a good 12-gauge shotgun - Remington model 870 is an excellent choice
... a 22 rifle - Ruger & Marlin have several great models
... a large-game rifle (270 or larger). I'm partial to 30-06 because of it's wide range of load capabilities
You could combine some of these by caliber. For example, I have a Marlin lever action in 44 Mag to go with my S&W Mountain Gun (or, do the same in 357 Mag).
Good luck and have fun!