Option A, by a longshot. Call me crazy, but I want Smash to appeal to a wide audience, and thus I think equal focus should be given to all aspects of each installment rather than hyperfocusing on the multiplayer experience and leaving the singleplayer to rot like Smash 4 did, which I think really hurt the final product more than most were willing to admit when the game was new. That way, anyone who really likes something that the competitive scene may not be super keen on - Story Mode or Break the Targets, for instance - won't feel alienated by future installments. Besides, this approach would probably lead to greater sales numbers from a purely pragmatic standpoint, even if I don't exactly agree with that line of reasoning.
Would you rather have Pichu, Squirtle, and Ivysaur all return (not necessarily with the same movesets as last time, mind you), but with another Pokemon cut to make room and no Pokemon newcomer, or just Decidueye get in and all the Pokemon in Smash 4 remain intact?