If we get both Isaac and K. Rool, here's the idea I've had for both trailers. The trailer's place will be the same as Megaman and Bowser Jr.
K. Rool trailer would start with Mario, DK, Yoshi, Fox, Diddy Kong and Samus doing a friendly fight, when suddenly they are interrupted by Kremlings. They quickly managed to defeat the Kremlings but then, 6 cannon balls attack the fighters, knocking them, and then K. Rool appears with his reveal catchphrase (probably sets sail or goes bananas). Then we have some in-game footage of him fighting several fighters and KOing the 6 characters of this trailer. We then go back to K. Rool laughing with all the 6 fighters on the ground looking at him and at the Kremlings behind him that prepare to strike. This trailer ends, but for now.
We know see the Smash logo, indicating a new character trailer and it resumes the K. Rool trailer. The Kremlings jump to attack the fallen fighters when suddenly, several pillars of stone erupt from the ground, sending them flying. K. Rool gets mad and he decides to finish the fighters by himself, when suddenly, Isaac appears in his front and uses his down-smash, earthquake to drive K. Rool back. K. Rool gets up and stares at him and then Isaac casts the Move Psynergy and we see the catchphrase Isaac makes his moves. We then get in-game footage of him fighting several fighters. The trailer will end once he summons Judgement and then there will appear a message saying that both Felix and Matthew are Isaac's alts.