New Vision for betterer background textures
Shifter adds some new stuff like alt-fire, improved enemy AI, unique weapons, you can also upgrade your flashlight aug to not consume any power, you can use aug canisters to upgrade the same type of aug you have installed, a Skull Gun aug, and some other stuff. However, it also lets you gain skill points from just killing enemies.
There's another 'expansion' mod for Shifter called BioMod, which is superior IMO. It doesn't let you gain skill points from killing enemies, and it also rebalances the augs greatly. There are now passive augs (like the one that saves up bio-energy, the aug itself no longer consumes BE), automatic augs (augs which you can toggle on and off, and only use BE when they have to be used, like the aggressive defence system that only uses up BE when it is intercepting missiles), and your standard active augs (press f5 to go fast). Upgrading the health regen aug now only reduces the delay before it kicks in, there's Thief-style mantling, Swimming has been replaced with Athletics, which determines your overall movement speed, and using computers no longer makes you invisible.
I'd recommend you get BioMod, the latest version of Shifter is already included in it, so no worries about that.
We're also waiting for Revision to get released, which the devs are not releasing yet despite it being practically finished for whatever reason.