When I first joined, and even now, I really enjoyed visiting abandoned bases. Sometimes you'll find little tidbits that hinted at what the faction was like when it was at its prime. Other times there's bases that were never finished and you wonder what it could have been like if it was ever finished. The history and the guesswork/assumptions involved with viewing it makes it a lot of fun. At the same time, there's lots of blatant griefed areas with no connection to any kind of base or otherwise, which do take away from the landscape. Griefs also happen within the abandoned bases, but I think that also can add to the "ruins" vibe when visiting, which isn't always a bad thing when exploring.
The landscape itself is very flat as mentioned though and the later made maps do look better and are more fun to build on thanks to terrain variance. I think Ceardia still has enough exploration value to be worth keeping a while longer, and potentially fixing up a bit in needed areas if people have the time available to do so. At the same time, I do acknowledge it looks very messy on the dynamic map especially. The problem with Minecraft in general is: you can't keep a "perfect" landscape without stopping people from building. Creating is the point of the game and everyone has their own artistic approach, whether it's a giant box castle or a simple yet elegant temple on a hill overlooking a spectacular view. Erasing one person's base which may be viewed as "ugly" by some, may have been that player's greatest creation and masterpiece to them. Judging it and destroying selected creations takes away from the purpose of a building game. There's some exceptions to that mentality, but that's my general stance on it.
I think a potential idea worth considering is not "how can we fix Ceardia," but "what can we do to avoid landscapes getting destroyed in the future?" A possible alternative is to make a designated RP world, where builders have to be approved, but anyone could visit and interact with levers, chests, etc like normal though. Moderating an entire world on a case-by-case basis would be a huge task, which his why that'll likely never be an option. With the above mentioned idea, the other worlds outside of this specified RP world would still be fully open to building like worlds currently are, making this location the only one with limited building. In addition these builders would also be able to self-moderate the world area, with the ability to take down incomplete (clearly abandoned) projects, report an approved builder who may be abusing the privilege through griefing, and help keep the environment nice in general.
There might be other better ways to approach the idea, but the jest of it is: the only way to avoid every world eventually reaching a semi-messy/griefed looking state is by adding some degree of restriction. That in turn defeats the purpose of playing the game, making the idea a compromise for that. The RP world could act as a poster child of sorts for the server, while there'll be plenty of other worlds where everyone can play like normal and there's no building restrictions as usual.