The concept of Battle Royale is fine but that of Season Pass sucks especially when Jagex tried to implement their own RunePass without putting the much need content there.
Last Man Standing is fine in OSRS. I don't see why not.
Something similar in the concept of survival of the fittest is fine for RS3 too. I don't see any reason at all such a new mechanic, if implement in RS3, should have any correlation to Fortnite or OSRS though.
So yes to Battle Royale game features in RS3 but no to Fortnite season pass or LMS as pretty much just a gold sink they perhaps desperately need in OSRS. RS3 don't need season pass and it is deflating nicely. Gold sink is just for a failing economy which doesn't happen in RS3.