Back to rifle design:
-I prefer bullpup layout. Mostly FN F-2000. There are some flaws, but you can work them out. The case ejector channel is to long, dirt will close it fast. Ice and snow? You can shorten it.
-Ammo: after a caseless design, like 5.56x41 LSAT CL, I came to another solution: normal case, cylindrical design, a 6mm bullet (5 grams) but with a new propulsion, like 2-component, liquid or gel. I came to this idea after seeing tests with a slim cylindrical case design with 5,56mm bullets, same overall length like 5,56x45mm and same power. Shorten the case, enlarge diameter slightly to 6mm, use the new propulsion. So you have a small (especially diameter), powerfull and light package. Optimize it for a short 440mm barrel. For an assault rifle you don't need much more than 2'000 Joule.
-The reason to drop the caseless idea was as follows: to unloading the rifle you have to install an unloading mechanism to, so in the end you have a much more complicated design with a sensitive gas-sealing. Only left reason for caseless is weightsavings.
-For higher magazine capacity: You can use quad-stacked magazines. With the small ammo the size of a 50 shot Mag will not be much more than a normal 30 shot 5,56x45mm Mag.
-Use a rail above for sighting systems, one in front below for grips, granadelaunchers, ... whatever. As normal today.
This would be my design for a time frame around 2015. I can send a picture of it, if someone is interested.