Ok to start lets elimate the sights, and use a full length top pic rail. Modify the front "tripple tree" and remove the top portion so it becomes a "figure 8" with a tapered/ramped centering nub facing forwards on top that both locates and helps to secures it the rail system. It should also be fully secured to prevent accidental removal or movement by a std hk handguard hangar and push pin towards the front to lock it on the nub. The real of the rail towards the rear is basically an extended MP5 style MLI rail using std Weldments. Hmm. This would need some thought. The triple is attached to the barrel only, to allow for free float and the sight moves with the barrel. if they are separated that would need a rework. For civilians, no problem. Combat would have an issue if dropped/damaged the sight may point right and the barrel left. Ghllie built a shorty with a rail from rear sight to shaved front sight. I really liked the look of it. He used two of my long pic rails welded to the tube.
The next big change IMHO should extend to the the cocking tube to make it user serviceable for changing its operation to the right side or for replacement. Make it float at both ends and locate it in place by a pin through the figure 8 at the front and by a tab at the other where it gets inserted in the reciever that locks it into a channel in the bottom of the pic rail, or alternatively pin it. This is a hard problem to work around. The tube is rigid to allow a straight back pull on the carrier. If it is loose the pullback will be horrendous and the forearm won't be able to lock in solidly (since the forearm pin goes through the weldment on the tube.)
The hg can use the readily available std knights type rail or slim mlok hg systems already in production. Check
The AR lower and magwell of this design seems reasonable and in keeping with future updates such as ambi selector etc. It has to have an ambi bho option that operates on the carrier and not the bolt head. This has been done before, and since we talking about the use of modern mags they will have followers that should work well. It should also be easy to engage and release w/o a magazine. The mag release should also be ambi in operation. This would require not using the HK style FCG pack system but, in the interest of moving into the future, ok. The BW20 looks like a SCAR lower was pinned on.
A nice option would be to have a forward assist and brass deflector. MSG90/PSG
Im less sure about the stock but lean towards a non folding but collapsable ACR or AR type stock.
Check
The muzzle should be threaded in std muzzle device patterns 1/2x28 or 5/8x24
US threading, check.
it would also be nice to have a progressive rate or adjustable hydraulic buffer instead of the spring loaded affair of the g3. Like a SCAR