What if it is a triggered ability that check on stat changes like:
Whenever <Cardname>'s Power or Toughness becomes 3 or greater, it gets an indestructible counter.
Whenever <Cardname>'s Power or Toughness becomes 2 or lower, remove all indestructible counters from <Cardname>'s.
That works. Triggered or activated abilities look at the game state at a specific point so they don't get tangled up in layers like static abilities.
Though entering the battlefield while an anthem is active might complicate that as it isn't "becoming" X or greater it simply always was.
While this looks like it works when you check under the hood it falls apart. With abilities that are constantly checking a status meaning they are "as long as X, Y" Y much come after X in layers. This unfortunately means you can't check p/t to grant anything since p/t is the last layer. Any abilities that checks p/t can't be static so it locks in the time its checking.
This is interesting and I've never come across it before (because they don't print cards that don't work, obviously) but it makes sense. With this rule you can't have stuff like "This creature gets +1/+1 as long as its power is greater than its base power" or other questionable stuff like that. I'll have to keep this in mind designing cards in the future.
It becomes mildly annoying when working between layers. You can't make all flying creatures blue or all red creatures elementals because those first things happen in later layers.
Oddly enough you can grant +x/+y by checking base p/t. So "creatures with base p/t 1/1 get +6/+6" is viable.
Apex Predator 1GG
Creature — Elemental
Hexproof from planeswalkers
As long as Apex Predator's power or toughness is greater than 3, it has indestructible.
3/3
While this looks like it works when you check under the hood it falls apart. With abilities that are constantly checking a status meaning they are "as long as X, Y" Y much come after X in layers. This unfortunately means you can't check p/t to grant anything since p/t is the last layer. Any abilities that checks p/t can't be static so it locks in the time its checking.
While I probably shouldn't feel this way I love that you think you can best "can't" with your own "can't". Its adorable, like toddlers going on about how one is president but the other one is double president and so on until they run out of numbers. Which is quickly, because they are toddlers.
Putting that aside, I like the concept though its too cheap and wordy.
Apex PredatorsRGW
Creature - Elemental Beast
Protection from Creatures and Planeswalkers.
If ~ power is twice its base power and it doesn't have indestructible, it gains indestructible until end of turn.
If ~ toughness is twice its base toughness and it doesn't have hexproof, it gains hexproof until end of turn.
3/3
Though entering the battlefield while an anthem is active might complicate that as it isn't "becoming" X or greater it simply always was.
Oddly enough you can grant +x/+y by checking base p/t. So "creatures with base p/t 1/1 get +6/+6" is viable.
Putting that aside, I like the concept though its too cheap and wordy.
Apex Predators RGW
Creature - Elemental Beast
Protection from Creatures and Planeswalkers.
If ~ power is twice its base power and it doesn't have indestructible, it gains indestructible until end of turn.
If ~ toughness is twice its base toughness and it doesn't have hexproof, it gains hexproof until end of turn.
3/3