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. Farewell will make you cry.

This build is aiming to limit the colors of the deck and get to the combo as consistently as possible. To that aim, I put in a bunch of rummaging effects to find Rata and Jaxis. Big Score in particular can allow you to run out both creatures on the same turn on turn 5.

The other part of doing the combo is not dying. To that end, I included a bunch of interactive and kill spells. I'm hoping the Temporary Lockdown will have relevant targets in the upcoming meta, but if not, it can be replaced by another sweeper. I chose burn spells that have the option of going to the dome, as one of the ways I can imagine actually winning the game is getting multiple Rem Karoluses into play with the combo (which can attack that turn with haste) and the difference in damage can be made up with a very amplified burn spell. Rem is also good against burn decks and potentially damage based answers to the combo (lightning strike on Rata or Burn Down the House to sweep). 

Fable of the Mirror breaker is just a good card, in addition to its rummaging effect. I wanted things for it and Jaxis to copy, so I included Bloodthirsty Adversary (I thought about Inspiring Overseer too).

Some other options I considered, but didn't include because they were 4 cmc, were Ognis or Atsushi. Ognis with copies can make a ton of (tapped) treasures, and a Fireball X spell could be included to use them. Atsushi can also make lots of mana or give a lot of card advantage or selection.

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 memey with all legendary creatures to get more potential value off Rata. I tried drawing up that build here: All Legends

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?