It sounds like you're defaulting to a rotating AR-type bolt, which makes it necessary to have a barrel extension to lock into with a QD barrel. You could lose the need for a barrel extension if you used a tilting/dropping bolt instead, like the BAR/M240 or Bren.
Also, if you want this to be something a decent home shop can manufacture, you should stay away from forged parts. Design the components to be simple to set up on a CNC machine - keep radii to a minimum, and minimize the number of fixtures needed for each part. Nice simple stampings would be a good way to go too. Castings and forgings are a lot more expensive for small-scale production.
Gunlab.net
i have been thinking about this as well, making it rather simple for someone to build themselves.. and actually, the simplest way i can think of to have a monolithic upper with a forearm as part of it with a full rail was to form the upper on a shop press.. shaping the top of the receiver, and top half of the forearm as a rounded section, bottom half of the forearm would be removable, and the remaining part of the receiver section would be straight, bent at the bottom and welded along the seam...
then i was thinking of having the rail spot welded to the bottom edges of this for the magwell and trigger group to slide onto
point of the barrel extension was to employ a quick change barrel feature, allow the end user to use any threaded, unchambered barrel blank for a barrel, requiring only the drilling for the gas block and the pins to hold it, as well as to support the top rail which will be part of the barrel assembly, so you will not lose zero when swapping barrels
at this point though.. im still trying to determine how the magwell and trigger group will attach... im not 100% sure im going to have them slide onto a rail from the rear... i may have the front of the magwell hook onto something on the outside of the receiver, and snap in, similar to the way an AK47 magazine hooks into its magwell which just require a riveted portrusion on the sides of the receiver and an AK style latch to hold it in place
reason i was avoiding hooking the magwell to the trigger group was that the magwell might end up with a little play to it after so much use... trigger group would do the same as parts wear, you would get even more play if they werent both connected directly to the receiver itself... so im thinking ahead on that part
one other thing im debating is whether this should be a .223 length action, capable of firing 5.56mm, 7.62x39mm, 6.8mm SPC, and anything else that can go into an AR15.. or if i should make it a .308 length action with a standard AR15 type magwell for the option of changing calibers between 7.62x51mm and 5.56x45 and anything else that would fit in it