Why not create an online hero battle simulator? It's obvious, isn't it? But get rid of all the unnecessary stuff, because it's completely different from the original, which, by the way, you probably wouldn't like yourself, because, as they say, it's no longer vanilla Heroes.
Instead, change the setting to something simpler and less time-consuming.
For example, use user-defined armies. Introduce points for each unit, spell, ability, etc., and let users start a game and specify the total number of points they want to use. The game finds an opponent with the same number of points, and they both choose their units and start the battle. It would be like Tactical Corner, but with different army compositions based on the user's preferences, and of course, in a pure online environment. And it would be easy to add some minor features, for example, that units of the same Faction cost (100 - hero level) % or something like that, where a fire mage buys 3 fire spells and gets 4 more as a bonus, chosen randomly and not higher than his level/5, and so on.
This would create a tactical simulator in the style of Heroes 3. And the idea of 2+ players in a battle can either be abandoned or rethought to fit this concept.
In any case, the strategic element in an online browser game looks dull, very addictive, and reeks of pay-to-win (((