I'm probably being more optimistic than history should deem feasible, but I don't think that the recent tweets really change much for Banjo's chances as DLC. We've always had to accept the following for Banjo to make it in.
- Nintendo and Rare have to be willing to work out a deal to bring the bear and bird back to a Nintendo console. It's not against Nintendo's interest if they want to sell N64 Classics in the future, if this is really a thing as rumors would indicate.
- Someone in Nintendo has to value them as being additions that would sell as DLC. The recent tweet doesn't say "We're going to put only advertisements in as DLC so you can look forward to some upcoming games!". It would be just as viable as a selling point for DLC to include iconic characters from a series that a certain generation of gamers grew up with. They very most probably have the Smash Ballot results as well from the previous years, which I imagine BK did reasonably well on.
- Sakurai has always said he would only add characters that would fit in Smash with a unique moveset (which is slightly conflicting with the amount of sword users but whatever). BK's moveset seems like it would be pretty unique. If there would be any conflict, it would only be certain moves from certain characters (aerials like Duck Hunt or DK, etc).
The main downside that is blatantly obvious is that BK, like it or not, is a pretty deadend series this last decade. I'm positive I've read things mentioning that characters that have a future are more likely to be featured. (Though K. Rool and Simon don't really stick to that "rule".) Their 20th Anniversary was this year and outside of some small merch, not much came of it. I don't doubt that Nintendo and Rare both like the series, I'm sure they do, but they also realize they're somewhat obscure characters in today's day and age on a genre that hasn't had a lot of representation. But hey, they were iconic for their day and age, it *is* their 20th Anniversary, and now's a good a time as any to honor or revive them.
24 hours ago I had the same DLC mindset as I do now. I imagine that there will be at least 2 first party characters, 2-3 characters that directly advertise an upcoming game on the Switch, at least 2 third party characters, and at least one character taken from the popular votes of past ballots or surveys. (Obviously given the 5 fighters in the Fighter's Pass, there will be overlap of these categories.) If we're hoping for Banjo to get in, I think the best bet is them getting in on account or combination of the past Ballot Results and advertisement of the N64 Classic.
If I'm being brutally honest, no, I don't think their chances are great. I think they're in reality not great at all. BUT I do think they're greater than 0. I won't let go of the dream until they release their final selection of characters.