Depending on what level of competition and scope you are looking for, here are some suggestions:
For a tiny group (dozens of players), see https://forums.sirlingames.com/
For a larger (thousands), but still moderately easy to learn, https://storybookbrawl.com/
For actual global competition (millions, good luck): https://magic.wizards.com/en/mtgarena or https://hearthstone.blizzard.com/en-us
I predict that 10-100k is the sweet spot, provided that there is some kind of competitive scene. So storybookbrawl looks like it might work.
Depending on what level of competition and scope you are looking for, here are some suggestions:
For a tiny group (dozens of players), see https://forums.sirlingames.com/
For a larger (thousands), but still moderately easy to learn, https://storybookbrawl.com/
For actual global competition (millions, good luck): https://magic.wizards.com/en/mtgarena or https://hearthstone.blizzard.com/en-us
I predict that 10-100k is the sweet spot, provided that there is some kind of competitive scene. So storybookbrawl looks like it might work.