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How is the ranking system planned – will it be the same for offline and online?

This topic is in the online section. Accordingly, it refers exclusively to online play. ;)

What about separating fixed-map and random-map players?

There are practically no fixed-map tournaments for Heroes 3 on the forum, except for the seasonal tournament, which is held only twice a year and has a small number of participants (only 10).

- FIRST LEAGUE - 10 people. Round-robin, one match each.

A round-robin format won't work; only a Swiss system will do, because there will be many games, and there will also be other tournaments...
If there is a second league, then any other tournaments will have to be forgotten!