Some of my ideas to fill in the remaining slots could be:
Assault Rifle Long Tier 2: FAMAS or Daewoo Precision Industries K2
Snipers from Tier 1 to Tier 4: Ruger American Ranch Rifle, Thompson Center Compass Bolt-Action, Ruger Precision Rifle, and Q The Mini Fix
All of these are chambered in 5.56 or can be, so they could fit. Just some ideas. I am really excited to try out what you have already made though.
On a side note. I would love to see you make a CZ Scorpion Evo 3A1, Honey Badger, and AS Val at some point.