Ooh...Caseless ammo. My tilt receiver suggestion is simple by comparison. The G11 didn't use caseless ammo just to come up with an ambidextrous bullpup, though that was a benefit of the design. The caseless ammo allowed them to achieve a specific design goal, the desired burst firing rate of 1800-2000rpm, by eliminating the extraction and ejection step from the firing system.
By eliminating the case, we of course can eliminate the need for aforementioned operating steps. It does indeed reduce the weight of the ammunition by eliminating the casing weight. Both noteworthy advantages.
However, eliminating the case raises some other issues which, along with the fifty round toploading magazine and expendable bolt group, compound its complexity of the proposed rifle. When you abandon the case, you are also eliminating the seal that prevents the hot firing gases from exiting the rear of the chamber. Now the bolt must obdurate the chamber against 40-60,000 psi of superheated gas with minimal leakage until the pressures have subsided enough to open the action safely.
Heat energy is another huge problem. While the heat evolved during firing is disipated by the metal parts of the bolt, barrel, and receiver, much of it is actually heating the casing which is then flung from the rifle, taking heat with it. Without these little heat sinks, that energy must now be absorbed by the system and be evacuated in the form of hot gas through an exhaust port in the rifle. Said port must now be positioned to clear left and right hand users, so its straight up or straight down. Dumping the bolt every 50 rounds isn't likely to cool the chamber and receiver very much. Accumulated heat will eventually cookoff the rounds, which was a problem that took HK quite a while to beat with the G11.
Unless you want a magazine like a Bren mag, mounted vertically upward, the cartridges will have to sit 90 degrees to the boreline and run along the top. To use a cartridge of any useful OAL, this almost necessitates the same rotary bolt arrangement seen in the G11. HK managed to solve the obduration problem and the vertical feed magazine simply by using a precision engineered (read expensive!) bolt and firing pin/chamber obdurator. The bolt must index nearly perfectly and be strong enough to survive multiple (50) firings with no leakage, yet be cheap enough to just throw away at each mag change. Lets not forget it must be light enough not to cancel the weight savings from eliminating the brass casings. And fifty rounds must fit within the length of the rifle ahead of the breach. That would be easier to achieve with a square or hexagonal cartridge cross section, so there is no wasted space in the mag. Of course, that means that the conventional linear bolt is all but impossible and the rotary design must be used.
The 4.7mmx21 caseless round selected by HK was designed to deliver compact rounds with low recoil, not for their terminal ballistics. They were intended to deliver their wounding effect from 3 rounds hitting in a dispersed pattern, so the relatively anemic round is less of an issue. If you want a .308 or similar caliber bullet at a reasonable velocity, the caseless rounds must carry more propellent. This means a larger in cross section and/or OAL, with the corresponding decrease in magazine capacity. It also means more heat evolved which must be dealt with to avoid cooking off the ammunition.
One could always argue that high volume production techniques can significantly reduce the cost of the replaceable bolt group in terms of machining and (possibly) materials. This is probably true, but it will be a tough sell to anyone but the military, and even then, as seen in the XM8 arguement, the weapon will need to provide an overwhelming improvement in performance to justify its adoption. Then there are logistical problems with replacing ammuntion, not to mention those with the ammunition production itself.
Seems like an awful lot of work and expense to make a rifle shorter. Should they find a way to solve the above problems and make it all work, you are likely to find yourself with a short, handy rifle that is awkward to load and has a lousy trigger.