Moving forward, I'm considering the following changes to cards I design:
State-Based Keywords I always preferred the old "is indestructible" wording more than the current wording. So I'll be going back to using "is indestructible". Or, I would, but I'm replacing it with a similar "keyword", invincible (cannot be dealt damage or destroyed). So that wording looks less lonely, I'm introducing a few more "state-based keywords": inviolate (cannot be enchanted or targeted), inevitable (cannot be countered"), and inexorable (cannot be blocked).
As a result, Mr. Progenitus would look like this:
Progenitus Legendary Creature - Hydra Avatar Progenitus is invincible, inviolate, and inexorable. 10/10
Other Mechanic Tweaks Much like the new "Hexproof from ~" and the old "Protection from ~", all four of the above "state-based keywords" can be tightened. Any of the following are possible:
~ is invincible by black.
~ is inexorable by creatures with power 1 or less.