Apparently it seems like alot of people think the Gunslinger isn´t quite together. OK.
I thought a flavor-appropriate additional feature they could have would be Bombs.
Not as many / not as powerful as Alchemist´s perhaps, since they do have FUll BAB/HD (though Deeds could possibly enhance their bombs), but enough to do something. To go along with this, these would be free just like Alchemists´, with Gun POWDER also being free. Of course, Alchemy is a class skill for them.
Besides chucking bombs like an ALchemist, which they would generally be weaker at (besides the Full BAB, so maybe longer ranges), I think it´d be cool to allow them to use multiple bombs to create very effective ´demolitions charges´. Could that use Engineering or Alchemy skill checks somehow? (perhaps the larger bomb you want, a higher DC must be passed, or the bombs have lesser effect?) That seems in line with pseudo-wild west flavor IMHO.
Thoughts? Obviously, adding extraneous 2ndary class features doesn´t itself fix a core dynamic that isn´t up to snuff, but this MAKES SENSE to me for them to have in the first place, and I think it gives them a slightly more broader/interesting role in a party (besides the guy with expensive weapons who kind of rocks until he needs to re-load/unjam his weapons).