Legacy's past
Back in the summer of 2004, September 1st to be exact, the lists for "1.5" and "Type 1" were split. Before this "1.5" was thought to be the bastard child of "Extended or 1.x" and "Type 1", when the lists split and the dust was cleared there was a new format, Legacy. When the lists were split many people were angry and outraged at what Wizards did to "Their format". While people couldn't all agree on if the change was for the better or worse they all knew Legacy was here for good and it wasn't leaving.
September 20th, 2004 the changes were now active to the format and people were now terrified of combo decks. Many terrified people grabbed all the Null Rods and other artifact hate cards that they could find and held them as guns. While people stalked their local metagame with these cards in their maindecks and sideboards they never ran into these ghost combo decks but a sudden surge in control such as Blue Bull Shit and Landstill sporting their favorite new toy Fact or Fiction. People feared a dreaded "Combo Winter of 2004" with all the new broken cards such as Lion's Eye Diamond, Lotus Petals and Moxen, but it was a warm winter and the combo was no where to be found.
As the year went on people were coming to realize that Landstill, ATS and the combo deck of the format Solidarity (Which didn't abuse any of these new profound unbanned cards) were powerhouses in the new metagame. "Gaea's Blessing all round" people would say as they shoved them in their sideboards for David Gearhart lookalikes and "Tormod's Crypt for all" for the disgusting card advantage that ATS was creating. Landstill was slowly being pushed out of the upper tier from not being able to deal with the combo deck and only having a decent match-up versus ATS.
Soon little green men swarmed over the format from its cousin Extended, these men were swarming metagames everywhere and weren't planning on leaving. These little green men and their Wastelands, Rishadan Ports, Lackeys and Aether Vials were the final straw for the decline of Landstill in the metagame. The crowds screamed "Ban the Lackey, No! Ban the ringleader" but Wizards heard no screams. Goblins was crushing the format and forced people to play removal on turn one in the forms of Swords to Plowshares and Lightning Bolt. During this period of time, both Solidarity and ATS were slipping on the radar but ATS was free falling without a parachute.
A few members of The Source, were brainstorming what it took to stop the hordes of little green men from overwhelming the format, it took cheap large creatures, multiple answers for a turn one Goblin Lackey and free countermagic. When people got their innovation and homework done there was Threshold/Gro/and NQG.
Big Arse 2 happened in Syracuse, NY in 2005, people played all day and when it came down to it there wasn't a Goblin player in the top 2!? There was a Gro player without the "Gro" in his deck but instead Meddling Mages. After big Arse 2 there was a solid three decks that formed a triangle that finally seemed like it was never going to go away. Threshold, Vial Goblins, and Solidarity were all claiming tickets for Christmas Town and they were not giving them away.
A year pasted and this triangle remained unbroken, Vial Goblins still the best deck in the format and Threshold still close on its tail. With newer sets coming out and newer cards such as Rite of Flame, Empty the Warrens, Jotun grunt, Counterbalance and many others Solidarity found its self the slowest and weakest of the Brady bunch and slowly drifted back to tier 1.5. Soon predictions of a "Combo Summer" would appear.

Present Legacy
Currently combo is thriving in the new metagame, pushing Goblins on its butt and is slowly starting to send those little green men back to the hills. Although, with Threshold's newest toy Tarmogoyf and Counterbalance/Top it's trying to hold the combo down keeping the little green men in the bay's of Legacy. With combo's newest toy Empty the Warrens being everywhere many decks are currently packing cards such as Engineered Explosives and maindeck Pyroclasm. As effective answers for Combo, Threshold AND Goblins many cards such as stifle are now being used because it counterspells Goblin Ringleader as well as Tendrils of Agony and Empty the Warrens. Decks like Faerie Stompy and Red Death are thriving being able to deal with the upper tier with positive match-ups.

Legacy's Future
With more and more combo coming into the format there will be a surge of control decks making those Goblins in the bay come back for seconds, and maybe thirds. Combo will continue to become stronger and stronger with newer sets, because what's fun and cool in standard is broken and obscene in Legacy. Threshold will continue to do well, but slowly have less and less numbers. There will be a new competitor that creeps up from behind in the form of Ichorid decks. I believe the format will become faster and faster and eventually resemble the former Vintage before Flash and the banning of Gifts Ungiven.

What do you believe the future of Legacy is?

***Flash never happened(Wink, Wink) so it wasn't included.