This doesn't get talked about nearly as much as what mechanics or characters we would want in a sequel. So I've decided to talk about what stages I want, and I'd like you to share your ideas! This is my idea for the whole roster of stages, so basically I think this would be a good amount for the sequel. I'll go into more detail for the ones I know better.
EDIT: Made a few adjustments to some stages.
1: Everything in the original, basically unchanged.
2: Hekseville: Gravity Rush x Journey. We know this was a concept. I'm imagining it'd be on the top of the clock tower, with space off to the side where cars and trucks will fly by as temporary platforms. The invasion would be the pillar in the background shaking and opening to reveal the pyramid from journey. The right side would fill with sand to create a permanent sloped ground. Not sure about hazards. The stage would be big, like Invasion.
2: Gibbet Hill: Tearaway x Murasaki Baby. On the top of Gibbet Hill with the catapult on the left and a few platforms that occasionally get moved around by a giant finger. I'm not sure what the invasion would be like because we know almost nothing about Murasaki Baby, but I imagine there would be lots of giant fingers involved.
3. Groznyj Grad: Metal Gear Solid x War of the Monsters. It would start inside the warehouse with the Shagohod in the background. Medium sized stage. The invasion is the roof and back wall being torn off by Kineticlops and Congar as they battle eachother. The Shagohod activates and begins to battle them.
4. Seattle: inFamous x Siren. Should be somewhere important to the story of Second Son. Space Needle in the background. DUP soldiers watching the battle and occasionally attacking. In come nurses and other monsters from Siren. Everything gets dark and only a certain radius around each character is visible.
5. Forest: Folklore x Warhawk. Takes place in the magical forest at the beginning of Folklore. Faeries visible in the background watching from between the trees. Suddenly the trees explode and a Chernovan tank with soldiers battles a Eucadian plane with jet pack soldiers. The Chernovan soldiers eventually get overrun by faeries that attack from the trees.
6. Junes: Persona 4 x Soul Sacrifice. In the food court of Junes. Small stage. A few of the main party members watching the fight and cheering. Suddenly Harpy appears with a horde of goblins and she begins to fight the party members. The store collapses and provides new platforms.
7. Dragon's Lair: Patapon x The Order 1886. Large and flat. Majidonga appears from the right. He stands there and attacks exactly like in Patapon, with the same telegraphs for moves. Suddenly Galahad's team mates show up and begin to battle it. The fight causes parts of the ceiling to collapse and provide a few simple platforms.
8. London: The Order 1886 x Puppeteer. Downtown steampunk London. Monsters drive away citizens. Moon Bear King shows up. He's huge. His magic rearranges the battlefield as he attacks the monsters and the players. He talks a lot.
9. Courtyard: Knack x Devil May Cry. In a courtyard downtown with rubble piles. Goblins attacking in the background. In come a bunch of demons who destroy the goblins. The patch of ground the fighters are on is transported to Limbo where demons attack the stage.
10. Native Fortress: Crash x Escape Plan. At the wooden fortress with the same traps from the level in the original game. Bakuki appears through a hatch in the wall and his traps begin to appear as he yells and shakes his fist at the fighters. This stage would have a ton of hazards. Medium sized.
11. Hills and Spills: Sound Shapes x Parappa. First level in Sound Shapes. Medium sized. Very very musical. Hazards in time with the music. Um Jammer Lammy shows up and starts rocking along with the music. The stage shifts around to fit her song.
12. Sol: WipEout x Legend of Dragoon. The 2048 version of Sol because, let's face it, it's way better. A long curved platform, sort of bowl shaped. Ships come by every now and then and smack any players on the ground. In fly a group of dragoons (or whatever they're actually called) in dragon form. They begin to disrupt the race with their weapons and magic.
13. Midgard: Final Fantasy x Primal. Takes place in Midgard. To be honest I know almost nothing about either of these games. I jst think they should both be included and would make a weird, cool crossover. Someone who knows these franchises should come up with a concept. (Replaced by Gaius)
13. Field: Flower x Shadow of the Colossus. The first level of Flower, with the tree on a hill to one side. At some point there's a shaking of the earth as the first colossus steps into view. He become parts of the level and is a couple of moving platforms. Occaisonally he atttacks the stage with his club.
14. Rupture Farms: Oddworld x Gravity Rush. The first level of Oddworld. Medium sized. Nothing much is happening in the background besides Oddworld denizens wandering around when the Nevi appear and attack. They begin to invade the fighting area. They can be destroyed by the players, which makes them drop AP, and they also attack the players. There will be a few types: the basic eyeballs, pointy spear guys, giant mosquitoes, and shooty flowers.
15. Winter: The Last of Us x Locoroco. We won't talk about the Winter aspect too much to avoid spoilers. All that matters here is that a bunch of singing, colorful blobs show up and they make The Last of Us a happy game. Yay!
16. Badman's Dungeon: Badman x Motorstorm. Takes place in Badman's dungeon, which starts out small and without hazards. As the fight progresses, a giant pickaxe digs out more parts of the level, causing it to dragually grow to Black Rock Stadium size. As it grows, increasingly more powerful monsters appear. Once it's at its midpoint in size, a block breaks and creates an opening for a Motorstorm race to enter. Cars, trucks, ATVs, and bikes all come screaming across the stage, hitting players and monsters alike.
17. Apocalypse City: Motorstorm x Jumping Flash! In the middle of the city in Motorstorm Apocalypse. Very dynamic and dangerous stage. Things are constantly moving and there are lots of hazards. There is a race but it's all in the background so the racers are never hazards. And then something from Jumping Flash! shows up and does something. I don't know what but hopefully someone here can fill that in.
18. Black Forest Castle: Fat Princess x Tearaway. The Castle from Black Forest in Fat Princess. Villagers are running around building machines and catapults and doors and things. A group of three wendigoes attack, fighting them in the background. Occasionally they'll throw a villager at the fighting area.
19. Stadium: Tekken x Crash. It's basically exactly the same a the stage from the first Tekken, big screen reflecting the fight included. In the background King and Roger are sparring. The ceiling crashes in and in comes Rillaroo and Tiny the Tiger, who begin to do a tag team battle against the Tekken fighters. TNT and Nitro crates fall in sporadically. Nitro crates explode on impact, TNT begins to countdown when touched.
20. Gaius: Shadow of the Colossus X infamous. Starts off on the flat area above the water where you face the third Colossus, Gaius. With one slanted platform below it. After a while, a fireball will shoot down the stage, causing a new one to form from rubble in the water. The Beast approaches and starts fighting Gaius. Sometimes their fight will intervene with the stage. Medium sized stage. (Credit to ShadowMan)