I made a rough draft on how Battle Tree would work while making sure they will fix the issues the current BF has, plus what Kaosubaloo said about "smaller thresholds".
Battle Tree will happen on the tree, and split into different branches (difficulties)
Each branch has 50 Trainers, the 20th and 50th are Battle Legends.
Normal trainers have 3 pokémons, and Battle Legends have 3 + extras (1 to 3).
the extras will depend on the weather, always be types that benefits from the weather.
The player will battle trainers from 1st to 50th and be sent to the next branch with higher difficulty after beating the 50th.
For the 1st branch, the 20th BL will have about same difficulty has Alola Champion, and 50th as Galar Champion.
Beating any trainer will reward the player with a small amount of BP.
Beating Battle Legends will reward the player with big amount of BP and Ribbons.
Ribbons can be used to auto-restart the run, but it will force stop any run at the weather change.
So at most, players can "AFK" for 4 hours.
Battle Tree Town would have various kinds of shop/traders
The normal BP one, Held Item trader for BP+Great Ribbon (unlocks when you beat Branch 1),
Because we are in tree, this tree produces various kinds of berries depending on the weather.
Berry Trader for each gen (BP+Master Ribbon), 1st gen unlocks when you beat Branch 2, to 5th gen when you beat Branch 6 (equivalent to BF2800)