This started as an off the wall deck idea. Then I heard someone took a similar deck to decent results in Legacy: Link!
The deck is based around doing broken things with Dream Halls. Like putting it into a deck. Dream halls lets you cast a card by discarding a card that shares a color. The other major parts of the combo came from the set Conflux; the legendary Progenitus and the eponymous Conflux. Having Dream Halls in play lets you cast Conflux on the cheap, searching up your playset of Progenitus and another Conflux; use the progenitus to cast each other, and if they get counterspelled or wrathed away you've got backups, and an extra Conflux to repeat with.
Which is all well and good if you don't mind waiting until turn seven or eight to finish them off. If you want to speed things up you need a quicker way to get Dream Halls into play, or Progenitus. Show and Tell is made to fit the bill; For three mana you and him both get to drop something into play, and the only way he can beat your Dream Halls or straight up Progenitus is if he drops an Aluren and combos you out in response. After that we can add in some mana accelerants, like Lotus Petals or Elvish or Simian Spirit Guides to shave the occasional turn off of it. Let's bang out a rough draft list:
4 Dream Halls
4 Show and Tell
4 Lotus Petal
~16 other cards, ~24 lands.
In the 16 other cards we can slot Brainstorm, Ponder, Force of Will and either another cheap cantrip, some more acceleration or maybe some more countermagic. What we end up with is a mono blue combo deck that's not that quick and not that impossible to disrupt; killing them by turn five is still a turn or too too late, and they can counterspell your Conflux or Dream Halls right off the bat. Or rip them, with duress or Thoughtseize. Yeah, Force of Will is nice, but you can't rely on always having it in hand. So how do we make this deck better?
I'mma gonna take a look at the back end first. There's a very handy piece of disruption for practically anything they might be doing: Obliterate. If you destroy all creatures, artifacts and lands, then they can hardly have a better set of enchantments in play than your dream halls. And obliterate is even uncounterable. Inasmuch as progenitus can dominate a clean board, we can still speed the kill up some.
If you search up False Cure, Beacon of Immortality, force of will and two Progenitus, then you can pitch the Progenitus to play False cure and Beacon to kill them regardless of their life total, with Force of Will backup. Can't ask for a much nicer kill than that.
I'd really like more than 4 Brainstorm, 4 Ponder as card drawing/digging. I'm not sure what else would be good in this slot though; There are a number of good blue draw spells out there. The linked article uses Ancestral Memories, which I guess is good if you've gotten dream halls out, but it doesn't help you find dream halls itself, which is a major minus. yeah, you can play it on it's own, but only if you're sitting around until turn five anyway.
On the other hand, this deck doesn't necessarily have to be mono blue anyway; if we went into Black, we could use Duress and Thoughtseize instead of Force of Will, and get your card drawing through things like Sign in blood. If you're brave, you can run Dark Confidant, but the occasional Progenitus flip will appraise you of costs not even Bob Maher would be willing to pay. You lose out in that being blue black makes you run those expensive expensive duals and fetches. And the card drawing isn't as good. You could also run green for the spirit guides and maybe Natural Order as well. Or... Or...
A couple other thoughts that came up; remember the Shoal cycle from Kamigawa? Blazing shoal discarding Progenitus gives you ten damage right there; if you ran it with man lands you could run that almost as a seperate combo. probably start your manlands count with Mutavault; the merfolk subtype might be important in case your opponent's Lord of Atlantis gives you islandwalk. The other thought I can't get out of my mind is Shining Shoal; the white one. If you can redirect ten damage, you can almost kill somebody. Not that many sources in legacy will do ten damage at a time (tendrils, for example, does it in increments of two).
I suppose I should bang out an actual list before I go...
4 Dream Halls
4 Show and Tell
4 Force of Will
1 Beacon of Immortality
1 False Cure
4 Lotus Petal
2 Pact of Negation
4 Crystal Vein
4 Scalding tarn
A little land light, and definitely a first draft, but that's all I've got. What, you expect me to actually test my bizarre deck ideas? The nerve...