I have three ideas here, only getting less plausible and realistic as they go:
1. Master Chief Finishes the Fight! This one has been discussed to death, but if your talking earth shattering reveal that still technically exists within the realm of reality(Microsoft is in the Smash Bros. credits now remember), this is the way to go. He's a rival companies literal poster boy, has zero Nintendo connection, and is adored by casual fans and hardcore gamers alike. Its just too perfect, gives me literal chills thinking about it.
2. Goku. I know, I know. But is there any better way to literally break the internet than to put Goku in Smash Bros? It would be the Smash addition to end all Smash additions. I mean literally, you couldn't ever top it, so it would have to be the last one ever, which means no more Smash games. I mean, where could you possibly go from there. Oh here's Goku, but now where putting in Toad. It just doesn't work. But the hype would be legendary. Also, Goku has a presence on Nintendo systems, they even made a special anniversary Famicom featuring Kid Goku.
3. James Bond. Now here's the real surprise. One of the most popular and best selling N64 games, virtually the only top selling game IP that Nintendo has ever published that is not represented in Smash Bros in some way. Even Banjo is here now. I literally can't fathom a bigger surprise then ending the Ultimate version of Smash Bros with the character that has been right under everyone's noses the whole time. It would probably be the most insane, complicated gaming deal of all time, but potentially, the perfect feather in the cap for the now certified video game hall of fame.
Bonus: Scorpion or Sub-Zero. If you want to talk about legacy, there are few game franchises with more legacy than Mortal Kombat. A game so popular and so violent that it literally led to the creation of the ESRB which changed the landscape of video games forever. If that doesn't warrant a spot in the video game hall of fame, what does? Plus, can you imagine starting off a smash trailer and suddenly hearing "Get over here!!!" Shock factor at its finest.