Battlegrounds is more or less pay to play. Only being able to choose between two heroes instead of 4 is a major handicap and that’s been heavily complained about by the community
Duels will have a paid mode too along the same model as arena. If the free mode is way more popular than the paid one you bet anything that they will go out of their way to find a way to squeeze money out of the free mode. The cost of arena relative to the rewards has also been probably the biggest hinderance for that mode gaining any traction so trying to emulate that would be a generally bad idea