My focus has been on standardizing with common calibers. For my wife and I it is 9MM, .223, and 20GA. I realize that 20GA is a bit of a departure from the normal 12GA, but the reduced recoil, and reduced penetration of walls in a home invasion scenario seems to make sense for us.
A battery of multiple firearms with interchangeable parts for a SHTF situation provides the best flexibility.
Agree with mic68 that upgrades that improve reliability and ease of operation are worth the investment.