For rush...

I think with sufficiently low minion blanket cost decreases, you can create a 'smooth looking' system where a 0 cost unit is actually something so powerful that it sucks to leave a hole in your army at 0 cost (unlike now, where not playing a minion does not have an especially high opportunity cost sacrifice). Rush builds don't even care about most of their minions anyway, if it's as slow as a Militia or Pawn it's just going to sit there for most of the game.

CEO also has these minions that are arguably 'well priced' for their cost (Phantasm, Ghost) but in reality have bad synergy with rush (even Ghost++ has mediocre synergy, being slow) and so they're priced maybe around normally for a regular build, but are basically upfront costs to play rush.

For fast development...

Blank units are really fun for fast development, I've done it for Minotaur & Behemoth synergy. This does have the risk of empowering White's initiative even more.

If you made a piece that said "ethereal on turns 1-3" this would make it more suited for fast development instead of rush (which wants to keep its ethereal threat), but it's no perfect solution. Maybe "can only be moved through but not attacked through".

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I think the game is ultimately healthier by forcing all minion slots to be occupied. This is another inelegant looking solution, and I do think being able to run blank slots are somewhat interesting, but it significantly buffs rush.

Also if you think of a blank slot as a minion, you have roughly the same number of potential armies (objectively, not competitively) if you disallowed blank slots and added 1 new low cost minion.

One thing everyone is missing about how TOTSTCEO deals with rush as well (BECAUSE OF YOU RYAN) is not mentioning Outcast pieces, which are 3rd row army slots. I think this can really slow down rush strategies, usually the best way (in CEO) to counter one with an average army is just to put lots of blockers at the diagonal. Normally it still sucks to play against if you have a high-mobility unit pinned and that can't get out the normal way due to the pin, but having a third row thing blocking the diagonal on turn 0 is really great against that. (Also a better solution than getting hard-countered by Favor Chess Cliffs, which screws over the attacker too much IMO.)

btw ryan isnt dark horse ur favrite piece in favuor chess, flight is gud mechanic

-main_gi