Jaxis and Ratadrabik are the combo. First, you can get down Ratadrabik (Jaxis can be first, but Rata has ward). Then, you can blitz Jaxis into play. Then, use Jaxis's ability, making a copy of Rata. That copy of Rata will die to the legend rule (and you'll draw a card to replace the discarded one). That will trigger the original, and you'll get a non-legendary token of Rata. Then, at the end of the turn, Jaxis will die to Blitz, and you'll get 2 triggers off the 2 Ratas, giving you 2 non-legendary Jaxises. If you manage to untap with that intact, you can use Jaxis again on the LEGENDARY Rata to double the number of Ratas (the Jaxis Pseudo blitz will replace the discarded cards). And with 2 Jaxises, you can do that twice. Optimally, you can go 2 -> 8 -> 32 -> 128 Ratadrabiks; although, you'll win sooner. It sounds very magical christmas land, but Rata does have Ward {2}, which helps it live the turn, and if you get wrathed, during this, the original is still legendary, so all the Ratas will trigger, and you'll get X-1 Ratas back. It's also only a 2 card combo. Farewell will make you cry, though.

This build is the memey full legends version, because I wanted to see what it would look like. My thinking was that the combo is a bit slow, so you'd want legends that affect the board or slow the game down. That list is a bit small in current standard: Lagrella, Toxrill, Ertai, and Thalia for the Tax. Toxrill is expensive, and I thought I'd rather have the 3 mana Lagrella over the 4 mana Ertai, pretty much every time. Braids, I'm not including, because the opponent doesn't need to sack, which makes it tempo negative in a deck that can't play any 1 drops. (Rem Karolus is potentially a good roadblock against Red Deck Wins, if that deck becomes meta.)

For this particular all legends build, I put it in a Humans shell. A lot of them are pretty good; you get ramp with Katilda, which gives you the possibility of playing out the Rata on turn 3 and gives the mana to play out the expensive curve; and you get access to Secluded Courtyard, as another 5 color land (Plaza of Heros is obvious). Thalia is the first choice to slow down the opponent and limit answers to the combo (imagine 4 Thalias on the board). Dennick is just a 2/3 lifelinker to help stabilize. Lagrella is basically a better Brutal Cathar, with the upsides of being legendary and having 3 toughness and potential to hide extra copies of legends under her to return against a wrath. King Darien is more wrath protection and can help close the game and can sack himself to trigger Rata when necessary. I think Jodah is bad and a bait, but I'm trying it over the third Mirror Box for extra memes. I considered Jerren as a decent ETB and pseudo grandeur creature, but I need space in the 3-drop slot.

In addtion to the 4 Ratas, there are 2 Mirror Boxes (should probably be at least 3) to get around the legend rule and 2 Roccos to find Rata as copies 7-8 of that effect. There are some more random 2 drop legends to get the 2 drop count up to 15 to not fall behind early; they can help end the game (or Giada has the word "Flying" on it).

The land base is high both to help with fixing and to have access to the channel effects of the Legendary lands, which are pretty good, if you can get them down to 1 mana. They are in there for spell effects that don't get taxed by Thalia (again to help slow down the game). 

I don't know what the format is going to look like, so I put 15 cards in the sideboard.

Another possible build would be more controlling, fewer colors, and rummaging effects to help find the combo. I tried drawing up that build here: Interactive Version

Jaxadrabik is the ship name I chose, because it has the same cadence as Ratadrabik, but Jaxabik rolls off the tongue better. And, when you draw a bad hand, you can say, "Jaxabik, more like JaxaBRICK." Thoughts?