All there basically is(off the top of my head) for the Emag and/or Xmag as far as aftermarket parts that AGD does not make is Blade triggers(the dont make them for the E/Xmag but do for mechanical frames) and custom grips. There are a few different people who make their own versions of the blade trigger and the custom grips, to find out more info goto www.automags.org/forums and ask.
Barrels -
Like with any other marker a good barrel can go a long way, dont think it's the first upgrade you need though, the stock mag barrel is great.
Bodies -
To my knowledge there is only AGD bodies made for the Emag. If your looking for a custom body, AGD sales the Slug body which many people and companies have turned into some nice looking stuff. Look into it since it has the same inner dimensions as the AGD mag bodies, it will not cause tolerance issues.
Bolts -
None of them do squat but the level 10 - If you dont have one, Get one.
Drops, and Regulators -
It's all personal preference but dont add any more Regs to your mag set-up.
Misc -
If you have a ULE body, that feedneck is Angel threaded, there are a bunch of different kinds out there so go ahead and get one if your unhappy with the stock one. I'd advise to only use AGD detents though.
**To my knowledge there are 2 different versions of morlock boards available for the E/Xmag but I do not recommend them.