I'm in love with Jesus and proud of it!
Make the Campaign less linear. Reach was way too stop-n-go with no room for map exploration/glitch exploiting,etc. That is why I enjoyed the H2 and H3 Campaigns so much.
Bring back skull-hunting. I don't like being given all of the skulls right off the bat. I want to earn them.
Make at least some armor in the armory unlockable through achievements and skull collecting.
Implement separate numerical and title ranks in MM and FF. For example, you could be a Field Marshall in MM, but only a Sergeant in FF.
Combine 1-50 with the current (or similar) rank system. Anybody can be a General, but there are 1-50 Generals, etc. Same applies to other ranks. A Warrant Officer can be a 50 just as much as an Inheritor can be a 14. Combine play-time WITH individual skill. Play time rank is based on hours played.
Get rid of AA's altogether. Sprint is OK, but it is the only exception.
GET RID OF THE CREDIT SYSTEM!