Swap it for a G22 and you'll be able to shoot 9mm, 40 S&W and 357 Sig by changing barrels. Maybe an extractor if you want to shoot 9mm a lot.
Actually, the G31 is what you should get if you want easy convertibility between those three calibers. The problem with starting with a G22 is that not all G22 mags are set up right for reliably handling .357 Sig, but all G31 mags are set up to reliably handle .40 S&W. So even if you mainly wanted a Glock in .40 S&W, it would be smart to buy the G31, and switch out the barrel to .40 S&W, keeping the .357 Sig barrel in storage for versatility. Now all your mags will work with either .40 S&W or .357 Sig.
Then, to add more versatility, you just need a 9mm conversion barrel, some G17 mags, and a G17 extractor and ejector.
