Birthright/Cerillia is my all-time favourite D&D setting. Here are the cool things I like about it and then some problems I think a reboot would face.
The more realistic/pretty overwhelming number of small states and petty territories all fitting together like a crazy jigsaw is a big change from the usual Points of Light With Blank Spaces set-up of other settings. It's all been explored, there is no unknown "wilderness frontier" for the players to rattle around in. The wilderness is already occupied. If you want it, then tell the DM that's where you want to have your territory.
And starting as a first level ruler of a domain does hit different. As well as their special extra different cool powers they get from their Special Blood(tm), many characters will start with a retinue, the sort of retinue an old school character wouldn't get until 9th level. So before you even start managing your realm, there's the matter of your bodyguard/household to take care of.
Then you have to actually run your realm. You can micro-manage it, split the responsibilities between the party (assuming you don't all want to start with your own petty nation), or find and hire advisors to make the job easier. But at the end of the day the meat of the game isn't kicking in doors and poking about adventuring. It's running your realm and politicking with other rulers, with an (admittedly big) side-order of undertaking missions to benefit and support your realm(s). And gold (measured in bars, not coins) is every bit as important as steel. Though the ever-churning events that you can't control and are always happening make strategising harder. Black Swan events? Birthright's got em all over, all the time!
The non-human species are tight! The elves are like the sidhe of the old tales, resent humans for their expansionism, and are not afraid of casually murdering them. Like how in some of the old stories, if you saw an elf you died. Or went blind, or whatever twist on the "don't go near the elves" moral the story had. Also the extradimensional-refugee creepy halflings are pretty much the only published halflings I like, apart from the old Mystara stuff. The existential threat from another dimension that ties in with them is also cool. Oh and the goblins are cool too! And the guys that fill in for the orcs are suitably scary.
Magic is special! By the time Birthright came out and especially by the time I got around to running it, I was soooo bored with D&D style magic. Wizard players have to worry about finding and accessing nodes to power their magic. And then worry about defending it from the couple of handfuls of other wizards in the setting.
The monstrous ruler guys are lit! And it's unlikely you'll be able to avoid dealing with at least some of them, and trying to fight all of them is a recipe for disaster.
So the problems? There are a few things that might rub some people's modern sensibilities the wrong way. There are so many moving pieces that running it can drive a well-prepared, skilled DM to absolute meltdown after a while, especially once the politicking starts spiralling out from the PCs. The players only really need to worry about their own realms (which is enough!) but the DM has to think about all the realms the PCs interact with, and then the realms that those realms interact with and so on and without discipline and focus it can get totally overwhelming.
Which brings me to my last problem, which is that a lot of the systems for realm management, especially the mass combat rules (which used sort of a card-based mini-miniatures system), are very much products of their time. Whole swathes of the rules that make Birthright different from every other D&D setting would need to be modernised, and likely written from scratch.
But man, Birthright is the best.
