So, Kamen Rider Saber DragonEagleJourney and Kamen Rider Espada LampHedgehogCerberus

There does not seem to be a rhyme or reason for any of these "full combo's"... like what?

They all share the same element. Presumably someone can figure out the significance of the chosen elements, but I haven’t.

EDIT: According to a guy on Twitter, Blades’ are based on stars (Pegasus is the eponymous constellation, Lion is Regulus and Peter Fan is second to the right and straight on til morning), Espada is based on bad luck and Saber is based on flying.

The weird thing about Brave Dragon and Lion Senki is that their elements are the opposite of Ryu and Shishi Origami in Shinkenger.

Kinda expected a phoenix instead of cerberus but why not. Maybe for Kenzan or a future rider. If not a phoenix at least some kind of holy bird. Maybe simorgh.

Because so far we have Dragon, Turtle and Pegasus, and phoenix/holy bird would complete a recurring quartet of beast from chinese folklore (pegasus often replacing the white tiger. I have no clue why like in Saint Seiya or BTX).

Gotta say, I'm really curious where the forms will go from here, as it looks like our three main riders will have their "best match" 3-book forms by Christmas! I know Calibur has an upgrade coming and Arthur fits in to Saver's arsenal somewhere, possibly with a dedicated combo eventually, but other than that I'm really not sure...
Will they just get another full combo each, or will we start moving onto more "unconventional" ridebooks for upgrades (along the lines of RT Sparkling, Trinity Ridewatch, Metal Cluster Hopper, etc)?

I'm unsure if they'll throw in a single book for a new form. They very well could.
Build's powerups used both driver slots, but I don't know if a three-slot megabook would work, unless it was a triptych of sorts.

And if they went the route of having one book signal a suit change, what would the other two slots be used for? Minor accent changes, like Hazard? Or would they simply not be used in show (which would be a missed opportunity, IMO)?

Then again, maybe with each major upgrade, our heroes have to complete the full saga before getting the whole thing. So a Brave Dragon powerup would grant a new base suit, but it wouldn't be at full power until later? But book hunting to complete each new form, and I could only see that happening for a major late-show upgrade.

But ideas that I'd like to see are a three-book powerup (probably a macguffin hunt, a la the search for the Argos Kyutama; EDIT could maybe go the route of having each book grant our main three riders a new body section and all of them combine for a suit), a new Emblem, or a new sword/driver. Oh, and the fabled theory of Touma writing his own story for his final form.

Bad luck? I don't have any knowledge on astrology or superstitions but how exactly does a hedgehog count as bad luck? The genie and cereberus I can understand to a degree.

i hope we get a 3 musketeer themed power up, or alternately there is this theme of past swordsmen and mentors so maybe saber will get like a buster themed power up or espada gets like a caliber upgrade.

It’s a Chinese thing.