Main areas:
Dripping Delta
Behemoth Fortress + Behemoth Fortress 2
Bedraggled Mansion
Ninja Hideaway
Bob-omb Factory
Carbonated Coastline

Smaller areas are also being thought about.

Guardian Bosses:
Petey Piranha
King Whomp
King Boo
King Bob-omb
Mollusque-Lanceur

Slorde Bosses:
Slario & Sluigi (Mario & Luigi)
Sliggler (Wiggler)
Sloom Boom (Boom Boom)
Slamek (Kamek)
Slom Pom (Pom Pom)
Motley Bosshorde (Motley Bossblob)

other bosses:
Bowser
King Goomba (might exchange this for something else that fits the grasslands theme- any ideas?)
Nabbit
Black Shy Guy Squadron
Ninjo (more on this eventually)
Stairface Ogre
Polluted Piranha Plant

Bros. Attacks:
Mario:
Bowser Shell
Leafy Leap
Fortress Smash
Iron Ball Roll-out
Casino Generator
Shell Shocker

Luigi:
Fire Flower
Piranha Cage
Whimp Brigade
Lightning Crown
Bob-omb Cannon
Drill Sling