Black Bishops or Knights might be Black Order in my opinion.

Maybe sth like this:

• Bishops: Black Order.

• Knights: Nebula, Ronan or sth.

• Rooks: Chitauri

• Pawns: Outriders.

I would change a few of these.

Id make captain america king

Scarlet witch or captain marvel.

For pawns id use shield agents, wakandan soldiers, mystics

The king pieces should be important but weak characters.

Well the king character should relatively be the leader. The king isnt weak, but a good leader knows he's of more value commanding from the stronghold rather than putting himself at risk in the battle.

True, but we’re talking about tactically strong vs combat strong. The queen has the most combat strength as the piece can move with relative impunity. The king barely has more combat strength than a pawn but I guess is the player surrogate and represents the tactical strength.

Yeah exactly. I mean chess is supposed to be symbolic about power distribution in a system.