I don't know that are there any mods like this, or have this been suggested like 1000 times. But here is my idea. Allow people make portals like nether portal, but from different block (like from cobblestone). And then when player uses flint&steel to portal frame, it turns to portal and teleports you to world connected to portal frame blocktype. When you teleport, it generates portal to other world and makes sure that portal isn't inside of stone or in the air(similar to nether portal mechanics). There could be admin could set like "/portalframe set cobblestone skylands" or so to connect block type and world. Particularly I would want to make portal to this world: http://dev.bukkit.org/server-mods/skylandsplus/ I know that there is now some ways to make this, but I want this to be really vanilla-like, so need to place signposts or use chatcommands to connect to places to each other. Just make portal frame and then flint&steel and boom you have portal to world. tl;dr Allow to make portals from different types of blocks and that block type always teleports to world which it is connected to.
There was a plugin that did exactly this (with the whole frame out of some other material), but I don't remember the name and I don't know if it's been updated in a long time. My plugin Nethrar (http://forums.bukkit.org/threads/tp...orting-nether-multiworld-portals-1-3-1.10874/) lets you set this up by changing the blocks in the top two corners (keeping the obsidian frame otherwise in one piece), and you use a config file to set what blocks link to what worlds.
