From NEOGAF good summary:
http://www.neogaf.com/forum/showthread.php?t=1044304&page=219 A new IGAvania
The largest IGAvania
- Extra boss
Releasing on PC/PS4/XB1/Wii U/Vita
Prequel minigame on said platforms and 3DS
Three playable characters
Nightmare difficulty
New game plus
Local campaign co-op
Async online features
- Stat lending, player ghosts for bosses, player graves, +???
Orchestrated music
Guest voice actor: David Hayter
Cheat codes
- Silly cheat codes
Bonus costume
Retro area
Speed run mode
- "Features to help speedrunners plan the perfect route"
Boss rush mode
Classic mode:
- Six linear stages, no RPG elements
- 8-bit music for classic mode
-- Two 8-bit tracks guest starring Virt
Challenge mode
- Online, co-op, versus; specially designed stages with extra powerful bosses
Roguelike mode
- Procedural dungeons, sharable seeds
Boss mode
- Playable bosses versus the heroes in a new area