I hope no one mines if I got into specifics on this, cause I think it's really interesting to compare with some of things tried, specifically on Mou since it's the only super customized I can think of (or at least that I have played). And honestly compared to your ideas it's actually surprisingly conservative.
And do you think this is something that is unique to being a mid-rate? As in low rates should be about traditional RO grinding, high-rates just for WoE and mid-rates for something different? Cause MouRO advertises as a low rate (although it really isn't).
What would you say about a midrate that went back to RO's roots and tried to change what was useless, to try and make them useful? For example. When you think of maps to visit on a midrate in RO, you most likely immediately think... the maps with the cards you need, the maps for good leveling according to your class, the maps for quests, and the maps of towns and PVP/WoE related things. Thinking on it, this leaves a lot of maps in the RO world that are simply untouched. Instead people go ahead and add in custom maps with the new BrowEdit which don't get me wrong, is more often than not impressive, still leaves me wondering why nobody thinks to try and spice up older, deader maps that already exist.
Like Skot touched on earlier I think there is always the problem of the over-optimizer. In this case it is people trying to figure out what the average exp per minute on a map is or where just the right things drop. In Mou there is this damn snazy difficulty system that lets low level monster level up and a random item drop system where anything could drop good items, in effect you could go outside of Prontera and fight some pretty nasty high level Fabres and do some decent leveling at max level. But that isn't going to stop the over optimizer from still going to Thor Volcano because of mob density vs how much exp per monster you kill. You can put the neatest twists on boring old maps but people will always go for whatever route they think is the absolute best for their goal.
Then I thought even deeper than that. The card setups for RO are pretty much always the same, what if someone took the time and the effort, went through the card database, and changed each and every card that is pretty much terrible and tried to give them some kind of edge? Wouldn't this open up a whole new world as far as builds are concerned, and.. if people are so hellbent on MVP/PVP/WoE, end up having a more enjoyable time? I realize people will just crack open the .GRF and see all the changes for themself, I'm actually not phased about that, because there'd be so many changes they'd be overwhelmed anyway. I think this would be an interesting twist.
There is a huuuge amount of cards though. There are so many variates of +1 stat cards and other plain garbage. If any card was slightly better than another similar card would you have to totally change that one with a a slightly less good effect?
I once thought it would be neat if junk cards could be consumable...but I'm not sure how that would work. But especially in a mid-rate where you are not stuck as a 1st class for lone who would really ever put a poring card on?
Then I thought about hats. Some hats are pretty cool (Hot-Blooded Headband, anyone?) but very limited as far as how useful they are. I went on and on and at the end of the day I wished such a server existed. I realize that's deemed to be outrageously custom heavy, but would it be custom in a bad way, or a new, cool kind of way?
The "solution" on Mou was just to have you be able to disguise your headgears, that way you could wear the generic best stat hat but have it look like the + 1 def decorative hat. (also has the positive of letting you do impossible combination, like a top hat on a top-middle-lower hat).
Not a real solution to the issue, but I wonder if stuff like cards and hats there are just too damn many to be able to perfectly balance them all. It's not like skills where in a perfect world every skill should be useful and it actually takes away from the gameplay if no one in their right mind would use a particular skill. Whereas you only having 10 choices instead of 50 for a headgear for your level is probably not something that would effect you.