Ok so this is something I have been running around in my head for a while now but hear me out on this theory. We all know the "Other dragon" that was shown as the "5th dragon" that battled the earthbound immortals 5000 years ago, this is the same dragon that Power tool dragon looks like. (keep that in mind for later) We never got to see it because the "5th dragon" turned out to be "Black feathered Dragon" and this is FACT the other signers said it was, and their marks reacted to it the same way it would when another signer dragon would. Also in the battle with scar-red nova (10000 years ago placing it 5000 years before the earthbound immortals) Black feathered dragon was shown fighting the scar-red nova. This leaves only one thing, Blackfeathered dragon must have some how died some time between the scar-red nova, and the rest of the earthbounds (I say died, more like sealed away or something like that, the point it it was gone.) Now where did it go? Crow got black feathered dragon, even thought it must have fallen some time ago, so where did this other dragon go? (to make things easier we will just go with the fannon name "golden gaia dragon" most people call it that.) So what happens to golden gaia dragon now? This is where power tool dragon come in, look at leo's playstyle, he loves equip cards right? I have looked at the golden gaia dragon and it looks like a "Dragunity" monster to me, which would make sense why it has a spear attached to its arm. (that mind you looks organic like all dragunity weapons) But what I wonder more then ANYTHING is why were leo and luna born as twins? I will cut to the chase, by looking at leo's playstayle he could play dragunity VERY well seeing how he allready has a vast history with equips, but more then that what if they were born as twins for a reason, Luna is a signer, and leo is not, but what if because they were born as a signer the "reincarnated signer power" was split in two, so rather then a 6th signer, its like two people that are each one half of the signer? this way Leo could have the golden gaia dragon and it make sense in the story. (this is backed up by the fact that G.G.D. appears in the new animes opening and ending along side the other 5 dragons.
Also I call into question the whole "reincarnated signer" stuff, by that logic, only the reincarnated signer can use the "5th dragon" so only goodwins brother could because if he was the reincarnated signer, then only he could use it. Crow has no connection to him so how can he be a "Reincarnated signer"?