Improbable Alliance creates creature tokens
Roaming Throne Listed at: $11 Roaming Throne is best for typal decks that have many triggered abilities because whatever triggered abilities of the creature type you choose triggers an additional time
I think that despite the second ability being worthless most of the time, having another Training Grounds is good enough that this creature will get some play, and it has the added benefit of being grabbable by a lot more of the kind of tutors you want to play in your deck than Grounds to boot
Esper Control is probably the third most-popular deck in the format right now, but is Chromium likely to be more of a mirror-breaker in the sideboard, or a versatile maindeck threat
Instead, they went a different direction and chose a Horror, which is pretty funny
What Land Gives No Maximum Hand Size