Given the mention of "Nothing to fear" and several mentions of Nothing in Empyrea, I suspect that the next world (or arc) will have elements from The Neverending Story.
Near the end of Khrysalis Grandfather Spider says, "Fly free, my children! Marry and make war! Build fleeting empires and towers of cards and beautiful disasters! Fly through the Spiral, to Polaris, Empyrea, and Mirage, to the Hidden Lands and the Broken Shores." Since we've been to Polaris, Mirage, and Empyrea I'd suspect the Hidden Lands and Broken Shores would be next (assuming they are not simply alternate names for Mirage and Empyrea).
I suspect Arc 4 will be the last main story line arc. After defeating the Nothing and presumably ensuring the Spiral will continue to exist there's not a lot left to do. By the end of that wizards will presumably be level 160 as well. That said, KingsIsle could then devote time and resources to side worlds. This would be nice because it would expand the Spiral out from the main story. Also such side worlds could have multiple areas that are split by level (as Grizzleheim and Wintertusk do), meaning low-level wizards can explore new worlds while high-level ones still have interesting challenges.
I do agree that the main story of Wizard101 is slowly crawling to a close. Though, to paraphrase Master Eraqus in Kingdom Hearts 3, "KingsIsle might surprise you" and a fifth story arc could be on the planning floor. After all, I am going to assume that the absolute limit of their creativity burns out when the level cap hits 200. I mean, MapleStory is already at a Level Cap of 275.
"Marry and make war?" Why not make love?
But to be honest, Arc 4 will probably be a bit of a shocking surprise. SkeleMystic believes that we will see the return of Spider Morganthe, who will be so powerful that the final battle of Arc 4 will be with her in a giant 16-Wizard battle where she has a million HP and 90% resist to all schools and immunity to shadow. (Okay, I made up the whole raid idea but you get the point - KI is struggling to keep the challenge up for higher level wizards without resort to huge amounts of cheats.) I can see Morganthe returning to be the villain of Arc 4, this time having powers that rival Bartleby and Grandmother Raven and slowly sucking the life out of them and withering away Wizard City into wastelands. She is now a goddess of destruction and can easily turn Merle Ambrose to dust with a click of her fingers. Remember, Darkmoor is where we battled the lich king Malistaire and he gained unholy tough powers from falling into the Shadow Web! Morganthe is waaay more powerful than Malistaire was and she fell in there too... and she learned the Song of Creation.
/That said, KingsIsle could then devote time and resources to side worlds./
I would love to have more side worlds in future updates, but make ones for all levels so that players can enjoy a break from the main story. Let's go visit some Pirate101 worlds like Monquista, Skull Island and Cool Ranch and make up some original stuff as well. Also, in the future, maybe add in a side dungeon in Aquila for Level 50 Wizards to explore between Mt. Olympus and Atlantea. I felt cheated that I had to wait four levels for a new challenge from the Immortals.
