Given what I learned from Eternal when we were talking about his FFT hacks, I know there's a massive catch to adding the Multiplayer items due to very few people here being interested in hacking the PSP version due to the problems with the extra (And half-assed at that) coding added to it. So, what items did you have to remove to put those into the inventory?
Makes me think Idea Factory got inspired by Square Enix when they decided to port Spectral Souls II to the PSP.
One of the reasons for most people here hating WotL is what I mentioned above: crappy coding by Square.
Others include people hating the changes on names and dialogues, which makes the game more akin to Final Fantasy XII (which takes place on the same world, only some thousand years earlier) and its "ye olde talke". On this one (about hate due to dialogue changes), I can say that the Tactics Ogre remake gets some hate for the same reason, despite being damn better than the original on a lot of things.
Not to mention that the slowdown fix is already available to download in romhacking.net by itself.
As for this: Multiplayer content: Eternal managed to implant some of it in WotL KO, but he had to replace items to do so. The way how the new content got into the game was done in a crappy way by Square Enix's part, due to them just slapping the PSX ISO into the PSP port and then adding content around it, made such a feat incredibly difficult. As of now, there is still no one able to crack into that content, especially because there is no way for the ones able to access the multiplayer content (though it is possible to access the new battles on WotL).
Extra characters: As far as I know, this is some heavy hacking job. It can be done as far as I know (not by me, of course), but I don't think you will find anyone willing to make this for the PSP version around here.