I'd like to see the ranks getting some meaningful effects on the game world.
For instance, if you have a very high rank with, say, the Federation, then when arriving at any new system where there are Federal aligned factions, your initial rep with those factions would start at friendly instead of cordial. If you're an admiral (or king) then certainly the factions of your aligned superpower would trust you a bit more than a total stranger no?..
This would allow the time to get allied and unlock the better missions to be made shorter, rewarding the players for their long time loyalty to the superpower in question. And alleviate the effect of moving to a different place and having to start over from scratch every time. While I believe that "local rep" should definitively be a thing, being high ranked with the ruling superpower should at least boost your initial rep with similarly aligned local factions.
Likewise, if you have a high imperial rank and flying in an imperial system, system security would arrive sooner when you're being attacked, as being a "person of importance" would make the system security prioritize giving you assistance.
Again, if you have a high rank you could also get small discounts on stations ruled by factions alined to the same superpower you have a high rank in.
I'd like to have the opposite as well, being high ranked in a superpower having negative effects on systems and stations ruled by factions aligned to the opposite superpower, but for that superpower ranking had to be mutually exclusive and that ship has sailed.
There are many small little things that could be done, requiring no additional gameplay mechanics, to make ranking up rewarding in itself by reflecting your current rank/status on the game world, instead of just being a numeric counter to unlock ships.
Overall I think reputation is very underused as a reward, for instance search & rescue currently has pathethic rewards and most don't give a fig about it, but they could give very high rep bonuses if you delivered the escape pods to their home factions in one piece (also, why aren't there any search and rescue missions?..), again creating another system where you wouldn't have to grind rep from scratch. Ater all you saved those people for almost no money, certainly that would earn a bit of extra initial trust, no?..