-I'm aware that Nintendo owns the rights to everything DK related, They obviously can add anyone from that franchise as they have full ownership, But it took heavy demand to show fans actually give a damn about King K. Rool and for Sakurai to take notice.
-Any video game character is capable of having moveset potential if you think hard enough, Some are just more easier and obvious than others.
- Just because Master Chief hasn't been on a Nintendo platform doesn't mean he can't join Super Smash Bros, It's up to Nintendo and Microsoft to decide if they want to pull off something historic and the unthinkable, There was even a Halo game in development for the Nintendo DS at one point and who's to say they can't revisit this concept or crossover in the future? If we thought Banjo-Kazooie was impossible, Having one of the most iconic video game characters in the history of gaming and the face of Microsoft's Xbox brand would be a absolutely insane seeing him face off against Mario.
Microsoft is slso credited on the copyright screen, Not Rare, If Master Chief joins they don't have to credit 343 Industries (Halo Developer) at the end of the day Banjo-Kazooie and Master Chief are Xbox characters and both developers are under the corporate umbrella from their parent company Microsoft.
The modern gaming landscape is changing Microsoft doesn't see Nintendo as a threat and have a healthy working relationship with them and to a lesser extent even Sony, They have their sights set on Google and possibly Amazon because they view game streaming as a service and the future and that's who they will be up against, Halo also got released on Steam and will have launch title for the next Xbox in holiday 2020 and even though it was a while ago and who knows if it's even true ,But did had those rumors of Xbox Game Pass and Halo possibly coming to Switch too.
If we get a 2nd Microsoft rep I have my money on Master Chief, Banjo-Kazooie already helped Microsoft get a seat at the table and I doubt they will reach back and go for a 2nd Rare character when Banjo-Kazooie was already the biggest character from that bunch and any other Rare character wouldn't be able to top it, But if Microsoft wants to top Banjo-Kazooie then they would go with one of the most iconic video game franchises of all time and the biggest IP in their arsenal and that is Halo.