I think the game could be simplified a little and not be ruined. You'd take some of the finesse out of the high level play but it'd make the game more palatable for a noob. Pro players will always find a way to do more and create a competitive advantage anyway.
Make initial build order system easier. Make the commander build at his full range instead of walking all over the place. A new player will lose so much build time if he doesn't issue a load of manual move orders.
Reduce the complexity of the manual reclaim. It requires a ton of map knowlege. It's very esoteric. Again not going to be popular with vets.
Get rid of 2.5 tier mex upgrades. Having templates and stuff makes placing 4 storages easy but new players aren't going to figure out how to do that. Give people a bit more metal storage to compensate. Maybe simplify the late game eco a bit in general.
Incentivise lower level players to host more maps somehow. You've got to give them bonus rank for playing a variety of maps. It doesn't have to be 20 different maps just more than 3 (not counting all the minor variations). You want them to progress into becoming average players not stay in a teamplay choke point map ghetto.
Bit radical but maybe give units more armour (would require gigantic rebalance). They die really quickly in this game which means paying close attention to unit position is very critical or your army can die super quickly. It takes a long time to reinforce.
FAF is balanced by pros which is a really good thing but it also means there's a bit of a lack of vision when it comes to accessibility. As the game continues to age it'll get harder to bring anybody new in so it's important to keep this in mind.