AR and derivatives are undeniably the best platform in almost every category. Ambi/left support is kinda a cludge, and STANAG is an okay magazine. That’s about the limits of the platform’s problems nowadays, the rest have been fixed
All the big advancements in the small arms arena are in electro-optics. Firearms have plateaued long enough that quibbling over a better gun isn’t the best place to throw R&D (or for that matter, procurement) money at. Buy all your troops RDS/LPVO for everyone, then Vis/IR laser.