1. New Game+ with multiplicative difficulty. A good story mode can go a long way towards replayability. (Depending on the complexity here, I could see it being a stretch goal.)
2. Boss Rush with a player level equalizer attached. Everyone is now Lvl 50.
(Pretty easy to just chain the rooms together and temporarily make the player Lvl 50)3. Arena - Part of the game as a sort of quest or mini-game awarding prizes. Also, a mode similar to Boss Rush with a level equalizer. (Something fun to do in the game without the threat of a true death for Hardcore, and a potentially easy thing to create compared to other tasks)
4. Basic infinite dungeon, slowly gaining various powerups but remaining a linear descent into the depths encountering the various terrains with slowly increasing progression limitations ensuring you explored enough to get the powerups. (Seems like it would be fairly easy to implement. Just restructure the level-generation algorithms and make sure each terrain type has basic rooms to go along with the ones that require special abilities)
(I think we're all agreed on those.)
5. "Classes" with restrictions through stat gain, gear options, etc. Sorcerer (Int favored, passive mana regen, banned from most weapons), Thief (Lck favored, banned from heavy weapons, high chance to gain rare items), Warrior (Str favored, banned from magic books, increased attack speed) (Very easy to implement as it just alters the stat gain and maybe flags items with a type that can then be equip-restricted)
6. Branching paths like Nethack would be a good approach for a semi-non-linear special-power-gaining storyline. On Mines Lvl 5, find the Ice Caverns. On Mines Lvl 10, find the Lizard's Den. On Lizard's Den Lvl 5, find the Volcanic Ruins. Each area needing some amount of backtracking so say, Mines Lvl 8 gives you the item to get past Ice Caverns Lvl 3. (I would much rather have the level generation algorithm set things up to give some backtracking rather than just descending into a bottomless pit. I found that part of Torchlight 1 to be very very boring as the only method of play)
Things that would make realistic stretch goals
1. Female Protagonist model sounds like a practical idea. (It's a very politically active feature. Seen it in the gaming news a lot these days. Genderswapping the main characters, etc.)
2. Customization for the player models, specifically in regards to hairstyles and hue edits. (Hairstyles would take extra animation for each different hair style, and hue edits would require an altered flexibility to the models as a whole)
3. Unique sprite models for the above mentioned "Classes". A Sorcerer/Sorceress would have a different look than a Thief. (Difficult to combine with 1+2.)
4. Chaos/glitch floors (standalone secret area or its own mode) - Full random floors that splice together the various tilesets and themes, or even single rooms that splice tiles and features in glitchy/chaotic/unnatural ways. (This idea is strictly for fun and lawls.)
4. Room creator / mod tools would be an interesting but challenging stretch goal. (I see positives and negatives to this. I almost feel like it would be better to have a fan-creation challenge where you approve a new set of rooms to add to the game as a DLC of freebie patch)
5. Multi-platform of course (3DS would be a fun fit for this game, though I could care less about the 3D aspects. Wii U lacks portability but provides an easier approach to map and menus.)
6. Remixes remixes remixes. Variety in dungeon crawling music would be fantastic. Instrumental shifts, tempo changes, etc. Every floor of the same area having the same music the whole area gets a little depressing after a while. And make sure there's some melody to these songs! Make music that sticks!
We're talking retro here, it isn't background music, it's a melody!PS: Hah. Werewolf transformation spell. I like that. It and other monster transformation abilities would be a fun gameplay approach for a 'Sorcerer'.
I guess some people liked it. 
