So one thing this could obviously be is the Meld/Host mechanic of Unstable come to black-bordered magic. Another mechanic I could see is this:"...you get to play with monsters, and you get to make monsters. It's one of the craziest mechanics we've made in quite a while."
"Combine <Cost>" means "<Cost>: Attach this permanent to target creature you control, and it stops being a creature for as long as it's attached. Combine only as a sorcery."
This plays in the thematic space of Godzilla, other kaiju, Voltron, gattai super robots, etc. It's kinda like a mix between Bestow and Equipment, like the former but less threatened by removal blowouts and so you have more freedom to make massive, crazy creature combinations. Some simple exampes as to how the mechanic could work:
Floatsac Behemoth
Creature - Mutant Leviathan (C)
Flying, Combine
Attached creature gets flying and gains +3/+3.
3/3
Adrenaline Slasher
Creature - Mutant Insect (U)
Double strike, Combine
Attached creature gets double strike and gains +1/+1.
1/1
Primordial Antecedent
Creature - Mutant Beast (R)
Combine
Whenever Primordial Antecedent or attached creature deals combat damage to a player, draw that many cards. Attached creature gains +2/+2.
2/2
I think this would really capture the feeling of "building your own monsters," like you're mashing together different Lego sets to come up with powerful combos.
