In my opinion, the shop function in PB:FttP & PB2 was under-utilized. It serves its purpose as a place to buy armaments, sure, but it feels a bit out of place as someone running-and-gunning everyone they see.
pew pew, pew pew, oh i finished the level, lets the buy the BNG, pew pew, pew pew
I also think that the credits were underused in both games, the purpose only to buy weapons. So you may be asking, "well what other uses could these credits have? its a 2D shooter, not some RPG". Which is a valid point, but as a player, I don't want to only use the money earned in levels (which is a bit confusing too) to just buy weapons and ammo. Here is where my idea comes into play.
Introducing an arms-supplying company which does business with independent groups along with large government entities would slide into the PB universe like a snug sweater. This company could be near omnipresent and sell conventional armaments but also performance-enhancing drugs or precision air strikes. They have no allegiance and demand a hefty price, but with good yet not guaranteed field effectiveness. They could also supply man-power but this makes less sense from a gameplay perspective.
Adding upgrades for battlesuits, like limited-use mobility boosts, and for small arms weapons, like infrared-integrated lasers paired with NVDs, would create immersive metas that would spice up the game quite a bit. Merging campaign and multiplayer funds together for players, along with sparse resource provision, would force players to use their credits carefully.
Using inspiration from games like Metro 2033 would be a nice addition too. So, what do you all think?