Depends on how large we're expecting here. All previous Champions and maybe some rivals? I'd say that would probably be enough to warrant the Battle Tree to feel less monotonous. It's also why I feel it's more or less important for there to be a mode for something similar to PWT where you face past characters exclusively if you do not wish to fight NPC after NPC.
That's the thing, though. Champions + Rivals only doesn't allow for enough challengers for such a mode. Part of why the PWT worked was because it had all past Gym Leaders, allowing for Type-specific challenges, along with the single-elimination tournament style. The Battle Tower is more of a gauntlet, with the 21st battle being the "grand finale." I don't see why they'd change that number, so if we go by that...
Gen 1's rival was also the champ, but let's add Red in there: 2.
Gen 2 had 1 rival and 1 champ: 2.
Gen 3 had 2 champions and 1 rival, 2 if you want to be technical on Brendan and May. For the sake of strengthening the numbers: 4.
Gen 4 had 1 rival and 1 champ: 2.
Gen 5 had 2 champs and 3 rivals... let's go ahead and count N as a champ too for good measure: 6.
Gen 6... if you want to count all the friend characters, 4 rivals, 1 champ: 5.
That totals to exactly 21. So if there was such a mode, where you only fought major NPCs the whole way through... well, there's no room for change, is there? There'd need to be a lot of different characters so each time you begin a challenge, it isn't the same 21 characters as before.
The PWT tournaments were much shorter and contained many many more characters, meaning that essentially no two tournaments were the same regarding who you fought.
If the Battle Tree
did include tournaments, they'd have to be just "Rival" and "Champion" tournaments; they couldn't have Type-specific ones as there'd be very very few Type specialists, for instance. Unless each of those Type tournaments used only generic NPCs, of course. But then we've lost what makes this Battle Tower stand out from previous Battle Towers, right?
Of course I don't expect Gym Leaders to be here, as much as I'd like the extra characters, but even just adding some plot-important characters can help up the number of total scoutable opponents while not feeling like it's "asking for too much."