Hi, this thread is dedicated to cards and combos you want to break but haven't figured out how to yet, reasons why they may not be "breakable" in vintage, and suggestions on how to break these cards.
Here's my list:
Djinn Illuminatus - This guy has so much potential but I haven't figured out where to put him yet. One of the first decks I posted in the improvement forum (maybe the first deck I ever posted) was an Oath list with Djinn + Pact of the Titan+ Altar of Dementia. The idea was you would replicate Pact of the Titan infinite times and sacrifice all the giant tokens to Altar and mill your opponent. It was a horrible inferior build but replicating cards like Ancestral Recall over and over again was really fun. I still think Djinn has potential somewhere though, his time will come.
Null Profusion- This has potential to be an awesome draw engine but Yawgmoth's Bargain costs the same amount of mana and is superior.
Enigma Sphinx and
Maelstrom Nexus - Cascade is really cool but in vintage these are the only cards I could see being meaningful in some sort of combo deck. The problem is you have to actually play Enigma Sphinx to get him to cascade and he costs

, you can't cheat him into play. Also you're very likely to just hit a mox or something that would be cheap or free anyway and if you hit a counterspell it's cascade was wasted. Maelstrom Nexus's problem is that it costs

and only works for the first spell of the turn. It would be cool to make a deck similar to hypergenesis (which is a deck that I wish was viable in vintage but oath or tinker/robot are just way better ways to get creatures in play) but having it be a storm deck using cards like Lotus Bloom and Ancestral Visions but you'd have a huge chance of drawing them into your hand as well and they would be pretty dead. Both of these cards are just too random. Mind's Desire is way better at doing things like this.
Dream Halls- I always loved Turbozhvi and was at first excited when dream halls was unrestricted but there really isn't a good shell for it to fit into.
Edit Here's a recent euro list that played dream halls though and top 8'd in March. It looks like fun and possibly competitive:
http://www.deckcheck.net/deck.php?id=25610Careful Consideration- This could be the card that has the most immediate potential on my list, it costs

more than thirst which is a drawback but you can still drain into it and it lets you fill your yard with welder targets or cards you could play with yawg will to storm out or shuffle back a DSC or Progenitus. The fact that it's supposed to be played on your main phase isn't a drawback because you that's when you'll be getting your drain mana anyway. Also it has +1 card advantage. Still not sure if this is a good find or not though, it's still not as good as fact or thirst. I would expect to see this more in a slaver build or something similar.
Megrim + Warped Devotion + Bounce cards + Urza's Guilt- I have a deck that abuses this with rituals and bounce and discard effects (like thoughtseize, Duress, Hypnotic Specter, and Dimir Cutpurse) with tinker/jar and is a solid shell for a budget deck but alot of the combo elements don't do anything on their own and there are just too many better combo and control decks. Hell of alot of fun to play though.
Three Wishes- Someone brought this card up in the "June 19th Banned and Restricted List Announcement" thread and I think there could be a place for it somewhere.
Rasputin Dreamweaver - Meadbert did a great job with his Rasputin Oath list but Dreamweaver in Oath also has many more possibilities I think. I just can't come up with a better list. It could also be abused in a re-animator style combo deck.
Thought Reflection - It costs alot of mana to get out but when I ran this in my old Panoptic Mirror-Replenish deck it was insane. I would love for combo to break this wide open, it just costs too much. Maybe in some sort of storm build that ran replenishes to get solid but expensive storm friendly enchantments into play (I don't know if it's that great of an idea but I might put a list together and post it in the improvement forums if it tests well).
Thrumming Stone - This card is totally and legitimately breakable and underplayed. I would love to see a good deck that abuses it and I think that's entirely possible.
Dimir Machinations - A solid budget tutor with a somewhat relevant ability if you decide to cast it and not transmute it (certainly not game breaking though). It finds a good deal of relevant vintage cards for

like Tinker, Yawg Will, Timetwister, etc. I'd expect to see this card more often if more low proxy tournaments were held and Long or TPS got popular again.
Perplex has similar usefulness but it can't be cast off a ritual like machinations can.
Show and Tell- I wish this card would be played more it's a great way to cheat things into play for cheap but I can understand why it isn't played. If your opponent has an Oath creature or tinker target or any other bomb in hand you're screwed. Also you have to have the card your cheating into play in hand already or this is a dead card.
Gelectrode + Wee Dragonauts These cards are pretty synergistic and Dragonauts can get pretty huge over a turn while gelectrode can deal a ton of damage and pick off opposing creatures (even Goyfs sometimes). Also Gelectrode is amazing with
Curiosity on it.
Niv- Mizzet + Curiosity - This is already broken but doesn't have a decent shell to fit into yet. I would love to see this get competitive. Maybe in reanimator.
Edit: I totally forgot about
Phage the Untouchable! She is unfortunately unbreakable unless you cheat her into play with Platinum Angel out or something, but man I would love to win a game with Phage.
I'm especially curious to see what people think of
Careful Consideration though.
Another card I forgot about was
Covenant of Minds. It's a got mana cost issues at

but it could be drained into and you can potentially fill your graveyard and draw 5 cards, it could have some potential in certain decks. However, I think most good opponents would probably just let you have the top 3 cards though making this little better than
Tidings.
That's all I have for now. If anyone has any ideas for breaking these please feel free to jump in and post suggestions and decklists as well as posting cards you would love to break.