See Era Test Creature. Considered at first.

The Epoch # would be varying from creature to creature. I picked 2 for this creature, but set ability would be as follows in the mechanics section: "Epoch N (This creature ETBs with N -1/-1 counters on it. At the beginning of your endstep, remove a -1/-1 counter from this creature. As long as this creature has no counters on it, it gets +N/+N)."

Why does this double down on the p/t boost? This could easily be:

>
Creature - Bird Warrior
Flying
Epoch 2 (This enters the battlefield with two -1/-1 counters on it. At the beginning of your ..., remove a -1/-1 counter from this creature.)
3/3

Why the big final pump and no power before?

I also wonder about the idea of removing a counter during the end step. This often means you basically immediately remove the first counter - making it appear less meaningful.

Obviously one could consider 2/5 up there rather than 3/3 if you want the same final p/t, but more power means once again more meaning.

The sample card basically functions as a suspend for the creature since the card with counters mostly is able to serve as chump blocker, so my idea also serves as a showcase where even the original mechanic can be improved by considering the other stats of the card.

Either way the "As long as this creature has no counters on it" ability seems like it shouldn't be part of the keyword - that way you can be flexible and sometimes grant a keyword instead of a p/t boost or make a more global variant e. g. "Creatures without counters on them you control have vigilance."

Interesting here is that you turn the yuga cycle on its head.

I think end-of-turn trigger came to mind first, because I thought of "end" in my mind rather than "beginning." I'd be fine switching that to upkeep.

The boost without counters came from my interpretation of representing the Satya Yuga part of the cycle following the Kali Yuga was that there should be not only a clean state (by which I mean a state where all -1/-1 counters are removed), but a boon effect as well (where the +N/+N with no counters came in). First, I could specify only -1/-1 counters as the absence of specifying counters was done out of shortening the reminder text. I will admit your alteration does have a bit more simplicity to it.

Alternatively, this could just work with a different kind of counter. Remove the last counter, and now the creature is bigger. By using both -1/-1 counters and a boost, tracking these creatures becomes complicated. But if this was a 0/3 that turned into a 2/5 in three turns... well, it's simple enough an idea to get across in one breath.