I just stumbled upon the balanced soldier rewards mod and remembered something from the Pokemon games: if you use a specific Pokemon tons of times, it gets hidden buffs that are not displayed anywhere.

This made me think that the more kills a soldiers has, the better his aim could be. I'm thinking in the ballpark of 1% aim per 10 kills, capped at 100 kills or 10%. This wouldn't make soldiers overpowered but would make you more attached to them and not be as easily replaced by "bought" high ranking soldiers.

Obviously, this would be a somewhat simple version. There's potential for more/different things like damage or even defense instead of, or in addition to aim, or even have it based on missions completed instead of enemies killed, or a mix thereof.

The only thing I'm slightly worried about is that it increases the snowball effect even more (do well -> do even better). I'd still subscribe the heck out of this mod.