What bottles would suit the Build riders when using their own BuildPhones?
Build: Lion
Cross-Z: ? (Touto)
Grease: Robot (Hokuto)
Rogue: ? (Seito)
MadRogue: ? (Seito)

And if A.I.M.S. had their own bikes based on V3’s, would you picture MetsubouJinrai riders’ bikes based on classic bikes like Kiva’s?

How about Thouser’s bike from ZAIA?

Cross-Z would use the Lock bottle.

Why the Lock bottle? And not just because it’s part of KeyDragon.

And Rogue? Any of the Seito bottles for his bike?

Gonna ask this, except for Wizard instead.

1. It's part of the KeyDragon best match, making it a good option for Cross-Z
2. It's lock and key themed, thus Cross-Z could use the key to unlock symbolically unlock his lock themed bike
3. It's not like anyone was using the Lock fullbottle. Sento gave it to Ryuga, but he barely used it, even with his sword.

I always thought it would have been cool if the Machine Builder used the Shark Fullbottle instead

I personally thought it would've been hilarious if he used the Engine Fullbottle or the Bike Fullbottle instead. The former making more sense, the latter being really silly to me.

Why doesn't Vulcan's Rampage Bullet use either Hopper? It was made using the data jacked by Thouser, and I'm pretty sure he took either Rising or Shining on one occasion.

The only actual reason is that the real world designers didn't want him encroaching on the title character's main motif. But we can imagine the Hopper stuff is like... extra encrypted or something.

I think the only data taken of the Hoppers was Shining Assault's? But that was put to use for Metal Cluster.