In the original BioShock weapons weren't made to be the focus of combat, as the role belonged instead to the novel Plasmids and Gene Tonics. In the sequel, though, they got expanded upon, and had an extra upgrade added to each of them. Nonetheless, they can still be vastly improved: certain ammo types and upgrades didn't work (such as the Gatling Gun's useless ricochet upgrade), whereas almost every weapon has a boring, space-filling damage upgrade which serves no purpose other than maintaining the status quo with the enemies' increased health.
Another thing that bugged a few players was the upgrade dilemma: in BioShock, some players didn't like the fact that they could fully upgrade every weapon in one playthrough, since it limited replay value. On the other hand, others were disappointed when playing BioShock 2 for exactly the opposite reason, as they were forced to go through the entire game (or at least a few save files) to see what every weapon felt like when fully upgraded. To solve this, I propose a solution that could satisfy both parties:
Simply put, each weapon starts with three basic upgrades the player can choose from. After picking two of those upgrades, the player can then unlock a third specialized upgrade unique to the upgrade combination chosen by the player. Example: a weapon starts out with, say, one blue, one red, and one yellow upgrade to choose from. If you pick the blue and yellow upgrades you unlock a unique green upgrade. Pick the blue and red upgrades, and you can get a special purple enhancement you couldn't have gotten otherwise. Each upgrade has a clear-cut role, and the third is intended to be tailored to the player's exact playstyle with said weapon. In the end, there'll be enough Power to the People stations to fully upgrade every weapon, but the upgrade wheel mechanic means you'll need at least four full playthroughs to discover and unlock every permutation.
This also means there'll be a slight change to the Power to the People stations: first of all, since they're going to be much more numerous (if every weapon is to be fully upgraded, that makes 24 PttP stations, twice as much as in the first game), getting to most of them is going to be a bit more challenging. Some of them could be well hidden, others might require that the player solve environmental puzzles to reach them safely; while others might spring traps for the player, in the same vein as the infamous Sinclair Spirits station in Fort Frolic. Secondly, in order to clear up the interface (eighteen upgrades all stuck together in a list was already bad enough), instead of there being a full upgrade list, it'll only show the weapons in the player's arsenal. Selecting one of them will bring up the weapon's full upgrade wheel, and hovering over each upgrade option brings up its full description.