Would help in teamfights, larger maps.
With the addition of ammo and possible recoil, as well as more grenade types, Pb2.5 would become a strategy team based game. A radar would be useful for tracking the last known location of an enemy. It would certainly allow for some interesting gameplay. Additionally, a radar could be used to locate dead allies or objectives, such as capture the flag. Let's say Eric decides to implement Capture the Flag in Pb2.5. The main problem we would have is that we would not know where the flag taker is. Take Team Fortress 2. When an enemy player takes your flag, your briefcase will point to them. If a radar in Pb2.5 is added, showing where the flag carrier is, it would be much easier to plan how you would stop them. In ranked/competitive, if your team knows where a few members of the enemy team are, they can easily get into position to attempt to take them out.
Recoil is also a good idea. Ammo and reload is being implemented, however it would still not have a nice gameplay effect. Adding a tiny bit of recoil would discourage point and click shooting. Although the recoil would basically be put your cursor down a little, since it is a 2d game and not like in csgo, it would still allow for even more strategic fights and makes the player think when to reset his accuracy/recoil, fall back, reload, etc. It would also disallow "perfect" accuracy all the time and spamming with guns. You would not have to worry about maps where bullets are coming right at you with high accuracy.
Feedback, suggestions on this? I think these two additions would be a crucial part to creating more interesting, fun, and stratetic gameplay. Probably more toward the Pb2.5 ranked/competitive side, but it would encourage more fair play, less spamming, teamwork and more strategy.