Some extra piecemaking tips:

1 - Avoid sidegrades. If there's a reason you'd want to use a lower tier unit over a higher tier unit that doesn't consider morale or cost (Greed and Princess are acceptable despite getting worse morale damage passives each tier), it's considered dubious design at best.

2 - Consider binding carefully. Neglecting to consider that a piece that can only move on D, or can move on D or A is doublebound and can only reach 1/4th of the board and removing its binding cheaply in the next tier is something I consider a design flaw. The chessboard pattern allows you to see at a glance if a piece has at least colorbound binding, if only light colored squares have movement options your piece is at least colorbound.

3 - +5 should be considered the maximum reasonable cost increase for a champion and +3 should be considered the maximum reasonable cost increase for a minion. Anything higher is a surefire sign your upgrade is doing too much. A +6 champion upgrade (Soulkeeper++) and a +4 minion upgrade (Apprentice+++) exist, but consider those just outside the line of what's reasonable.

4 - If your piece is solely a combo piece like GravityMage, people will forget your piece exists or meme the hell out of a broken combo.

5 - Avoid putting abilities that can destroy or render unusable units on Range 4 forward or further as Unblockable or Range 3 forward or further as Magic.