I know Fortnite isn't a bad game, although it is one of those games I feel too old to play as, with most of its playerbase being actual children, and the game being a lot faster paced/tactical than I imagined. My problem with Fortnite is that I don't see it as having a long lifespan, it currently seems like a fad. It's no cultural zeitgeist of gaming, but a just a mark on gaming history, like when League of Legends was relevant. Fortnite content in Smash Bros would definitely leave a dated mark on it more so than any other franchise I can personally think of. It wouldn't be bad, but it would be like a South Park episode: good for the present of feeling culturally relevant, but in a few years gives a weird dated feeling, and not the one of nostalgia.
Although I do agree that I've never been too much of a Rare fan myself, but there still is a huge crowd out there of people who would sell their children's souls to get Banjo in the game. I can understand the disappointment of having a company never doing anything with the IPs you once held dear. Although characters like Banjo would fit in perfectly for Smash Bros as a playable fighter, but sadly his series isn't going anywhere anytime soon.