I really dig the Kriss Vector's design. It's modern, practical, and looks the part. Several games have used it as a basis for futuristic SMGs. Obviously, it can't really be used for long ranged weapons as it makes them impractically long. It could have a few hypothetical improvements that may or may not be practical due to technological or marketable constraints. But here I am suggesting them for a hypothetical future firearm! This sort of stuff is generally good for artists when designing props.
Electrically fired ammunition: Less of an extreme shift than caseless ammunition, this still has a casing but uses an electric primer rather than a mechanical one. This allows the bolt to be significantly shorter, reducing the necessary size a lot more than the bolt's shortening would suggest. Note on bullpup weapons, the magazine is significantly further forward than the back of the stock; this is to accomodate the fairly long bolt moving back and forth.
Electrically fired ammunition would still benefit from the casing's ability to remove heat from the weapon, but also be safer and a lot simpler mechanically. Hypothetically, the electrical charge for the next round could be provided by the recoil energy of the previous round, but that sounds like it could be unreliable.
It is also a sort of ammunition modification that could work with existing weapons, allowing for retrofitting of previous firearms. This makes the ammunition more broadly marketable. I'd suggest the 5.7 mm as an appropriate testbed for this sort of ammunition, as it is a calibre that is popular enough, but not that widely adopted.
Ambidextrous Ejection: Less of a problem on the Vector than bullpup weapons, but I have a personal preference for forward ejecting weapons. It just seems like a neat way to have everything. Obviously, it can also be the source of jamming. This would have to be addressed by people who are actually engineers.
Under-barrel Magazine: A lot of future firearms experiment with odd magazine placement. My preference is directly below the barrel, similar to the Bizon SMG (not necessarily helical). The magazine can thus be held constantly, allowing for rapid removal as well as support. The magazine could also incorporate magnetic loading entrances, as seen on Apple products, but perhaps not.
Anyway, that's the fluff for a weapon in the setting I am writing.