You may find out that the "improved" AR-15 is what we have right now in the M16A4 and the M4. Increasing the caliber of the cartridge might be a good idea in some situations however.
Of course, if you want to own a piston driven upper with interchangeable barrels just so you can be different, then more power to you. I wouldn't say it's "improved" however.
While interchangeable barrels are neat, don't you have to rezero your rifle every time you change the barrel? Say for instance, from a 14.5" M4 to an 18" precision rifle. Each configuration might have different optics or sights that will need to be zeroed.
If you stick with the tried and true uppers rather than interchangeable barrel system like the MRP or Masada, you don't have to rezero for each barrel. Say for instance, a 14.5" M4 with a red dot scope to a 20" precision rifle with a 10x scope.
Alexander Arms and Sabre Defense make 6.5mm Grendel uppers for around $800 to $900. The Grendel has better ballistics than the 6.8mm SPC and 7.62x39mm. I'd suggest doing some research on the Grendel. It's a better cartridge than the 6.8mm SPC and the 7.62x39mm.
The Robinson Arms XCR is available in 6.5mm Grendel as well. It's over $1,500.
What do you think a gas piston will give you that you don't already have? You'll have less accuracy (read the reports on the H&K 416 in regards to it's accuracy as compared to the M16 and M4) and you'll spend near twice as much for an unproven system. The tried and true M16 and M4 gas systems have been with us for a long time.
The recent Army "extreme" desert dust tests with the M4 versus the H&K 416, H&K XM8, and FN SCAR had the M4 in last place. News reports loudly proclaimed the M4 to "perform poorly." Here's a few observations:
1. Milspec magazines are woefully outdated. How would the M16/M4 perform in the dust tests if they used Magpul Pmag magazines rather than the crapola milspec aluminium magazines? At least the Army could have installed Magpul anti-tilt followers in milspec magazines.
2. From a practical perspective, the test means that if you get into a firefight in a sand storm where you fire 600 rounds of ammo, you'll have about 10 stoppages with an M4. With the H&K 416, FN SCAR, etc. you have around 5 stoppages. Of course, this is assuming this data translates into the real world accurately.
3. I would still feel comfortable carrying an M4 since firing twice the amount of ammo I can carry with me in the middle of a 2.5 hour sand storm with no additional cleaning or lube seems like a fairly extreme test for any rifle.
A gas piston would allow you to use a folding stock.
LMT makes a great collapsible stock. The "SOPMOD stock" has an improved cheek area so you can maintain a proper cheek weld. It's not overly large and has compartments for batteries.
I don't have anything to say about integrated rail systems because I don't care for them. I don't need flash lights, laser pointers, and such hanging off my gun.
You can mount a forward grip on a small rail mounted to standard hand guards. MidwayUSA sells the "Falcon Industries Picatinny Rail with Rail Covers and Mounting Hardware" for $15:
http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=191715&t=11082005
If you want to improve your AR-15 buy some good magazines such as Magpul Pmags or milspec magazines with Magpul anti-tilt followers.
