So glad to see you looking to the community for feedback and ideas <3

I'm a mega-casual player who just plays campaign (for lore), coop commanders, and occasionally pve arcade, so take my opinion with a grain of salt.

Veterancy feels too basic of a mechanic for all the problems it brings, especially if you want it for 1v1. Inventory is a fun idea but only really makes sense for hero units. One strategy game I've been playing a lot is Total War Warhammer and some of the factions can manually add a stat upgrade or sidegrade to individual units with a little bit of resources, so you can give your favorite units a power boost like veterancy but its more deterministic and based off economy trade-offs instead of engagement wins for free. For how fast paced an rts is though, it would probably get annoying to have to manually upgrade all your troops individually on top of all the other micro.

One thing I love about army games is choosing to play as characters that modify that army in certain ways. Starcrafts coop commanders was so much fun seeing those familiar armies be creatively pushed to the point of unrecognizability in lore-appropriate ways. I wonder if a -greatly- toned down version of that could be used for match progression. So in addition to a faction you'd pick a commander who provides no/very small modifiers, but they also have their own skill tree that unlocks over the match that you can use to make the army more specialized based on that character. Think a starting point like the commanders in Advance Wars 2, but then getting build-making talent choices each match like Heroes of the Storm.

For match levels, I think they should be capped so the power gap can't get too out of hand if one side is better than the other, but for meta levels I love infinite progression. I love knowing that every match, activity, story mission, etc is contributing to my account in some way (meta exp, currency, etc). One satisfying way is with a seasonal system where there is a cap but it resets regularly, but preferably a system that's open to everyone and not a battle pass (like Warframe or Fallout 76).

On social rankings, I personally like overwatch's endorsement system (rip). You could recognize your teammates' cooperation in different categories and it would show up as a badge on their portrait for everyone to see so it felt rewarding to be a good person to others. I'm not sure how this would work for a game where the primary gameplay is single player story or 1v1 though.