UO had it right.
Alternate rule sets on servers are is only way IMO.
+You need PVE friendly servers -
-Only voluntary PVP under controlled conditions (like battlegrounds/arenas/duels), minimal grind, very solo-friendly. The EQ2, WoW, LOTRO type of "normal" servers. It's obviously popular.
+You need "hardcore" PVE servers -
-Only voluntary PVP under controlled conditions (like battlegrounds/arenas/duels), slower leveling pace due to things like XP loss on death, slower gain rates for skills, less XP for quests / killing mobs, etc. More difficult mobs (more health, more damage, etc) to encourage/enforce grouping and more difficult soloing
Travel options SHOULD BE THE SAME. There is nothing "hardcore" about having to ride/walk for an hour to get somewhere. If anything, UO had the most "EZ MODE" travel system ever and it was the best. You could create Runes bound to a location and teleport their at will. No mage skills? You could buy and use Rune Scrolls and Teleport Scrolls.
+You need PVP friendly servers -
Same as "normal" PVE servers, but also with open PVP between rival groups/factions like on WoW PVP servers
+You need "hardcore" PVP servers -
Same as my "hardcore" PVE servers but with the PVP rules of the "normal" PVP servers listed above and limited item loss on death. If you have 12 slots for armor+weapons+items, make it so only 3-4 or so can be "protected" from player loot with an insurance type system at a financial cost to the player.
This only works well if you give XP toward leveling for PVP actions/killing other players.
Roleplaying versions of these servers types are also needed.
Now, splitting your servers has risks and rewards. You need to have a very large population in your game in order to make it work. Otherwise, the more "uncommon" servers like the hardcore servers and the Role Play servers will be sparsly populated. Obviously, you can play around with server merges and creation/removal based off of player demands to achieve an acceptable level of players on each server type.
Ultima Online was doing this way, way back in the day. The Trammel / Felucca split was pretty much like playing on two different servers by today's standards, and they had Siege Perilous for their "hardcore" server.
Obviously, the rules and numbers are debatable, but the point is give players options.
But who would play on the "hardcore" servers? Why not just play on the "normal" servers and grind/farm/power level yourself to the top and then dominate the game? Why go through the hardship of the more difficult servers?
If you are asking those questions, then you belong on the normal servers.
The point is, give players choice. Freedom. You can't create a "hardcore" game and expect it to do as well as the more "mainstream" titles. It's just not going to happen.
You also can't continue to make carbon-copy cookie-cutter games that so many players find "too easy."
What you CAN do is create a game that has server rules for both. Enjoy.