There is load of mods (or rather stats) that have super limited use on the moding bench.
One example of that is the magazine capacity stat. I would combine fire rate with magazine capacity stuff, for example +60% fire rate +60% magazine in single mod. Simply for the reason that fire rate increase DPS, but it mess up your ammunition economy so you rather got for pure damage mod instead. Magazine capacity is basicaly reverse of that, so combining them would be really good solution.