if you like your 21c, consider the 30S SF. slim slide & slim frame. and your 21 mags will work in it too. ...
This is an obvious place to start. In addition to 1-way mag compatibility, you have the same manual of arms, and ammunition compatibility.
If you want to branch out in calibers, then look at the G19/23.
Keep in mind if you go with something other than a G30 variant, then you'll have more mags to buy, more ammo if it's in another caliber, and possibly another system to learn if you pick something with different safety mechanisms, decockers, DA/SA, etc.