Unless someone can completely crack the game open to the point where people can add content that isn't just more items, edited versions of existing enemies, and new quests, I'm more than certain that this is impossible.
You'd have to make new level maps, create several different routes of that one map, place down boxes, devise whole item droplists for all four difficulties, make field traps, make Ultimate variants of everything, etc. etc. A whole lot goes into making a single environment, especially if you're aiming to get the same quality that the original maps have, and depending on what you want, you may need to make a single huge map (if you plan on doing something like Crater, which I'm pretty sure has all four routes contained in a single map), or 2 or 3 smaller maps (based around setpieces, which is slightly easier to design, since you can piece mostly everything together), and that's for each area you want to have in a theoretical Episode 5. You'd be looking at inserting at least 5 to 10 different maps depending on what you're designing for an episode.
And if you're making new enemies, you'd have to design new code to handle their abilities and change THAT code for Ultimate, too.
Then there's the bosses. If you want a completely original boss, that's even more enemy code to design.
And there's also a story you'd probably need to put in, even though I'm sure none of us play the story anyways outside of getting certain quest-related rewards from it, since we've all played it to completion several times over. Though if you can make it more interesting, there might be a few people who'd be engaged in it.
So yeah. You'd be looking at a very, very daunting task. You'd need to get a whole team of people to do all that, and even then the problem with adding THAT much new content still stands.
EDIT: Also, considering nobody's tried doing that yet, not even the servers that add custom items into the game, it's unlikely there'll ever be one. If it ever happens, well, we may just see something that could be a contender for new recommended server (assuming its leaders do things responsibly.)