I'm of the opinion if I don't have the caliber I want it.
I'm the same way. You never know what caliber will or will not be in stock, especially nowadays. I own more .45's than any other caliber and its nice to have the ammo compatibility but variety is also nice.
You really can't go wrong with any of the options you listed. I would advise you to first decide on what caliber you would like. Figure out if you'd rather have that ammo compatibility or if you'd like some variety, then go from there.
If you go with the variety option I would recommend a .45 since its on the opposite end of the spectrum compared to what you already own. Out of the pistols you listed in your post I can personally vouch for the HK USP .45 (wearing it in a thigh holster at work as I am typing this). It is truly a great firearm but I would not recommend it for a CCW.
Have you considered the G30 instead of the G21? I use it as both a CCW in a IWB holster and as my nightstand gun. Glock now makes what they call the G30S which has the same capacity as the G30 but with the G36-sized slide which will make for much easier and more comfortable CCW. Perhaps that is something to look into.
Anyway, like I said, decide the caliber then go hold some guns.