Based off how Battlerite is doing on steam, one can assume if this is done by a better company it would be more successful. Now let’s explore some ideas on how this would work without changing too much or asking for too many extra resources/time.
5-20 man tops, I would go with 10 man to not hurt the other game modes too much. Could even be 5 man I think it would still be enjoyable and keep it on a small map.
You would find things on the map like +100 hp or %5. This could be scaled for damage, armor, spell armor, atack speed, etc.
Could find random skills or even ultimates, maybe even let the heroes find 2 or all the skills would be ultimates. This would certainly bring in players to try such madness.
Could make the royale start the game off with 4 ultimates and only find + health, heal, damage, or nothing at all. I would still play it.
Could make it all random heroes.
Leaderboards/ranked should always be a thing to keep people playing it.