Highly unlikely. The mechanics for Sma5h might be different than Sm4sh, but we'll have to wait and see. Melee felt nothing like Smash 64, and Brawl felt nothing like Melee. However, Sm4sh does feel somewhat similar like Brawl, but with far more improvements. It wouldn't surprise me if Sma5h felt like Sm4sh.
Now, will we get Classic Mode as we all remember it being before Sm4sh changed it? No idea. Hopefully. Will Adventure Mode return? That would be awesome, but I imagine it would be difficult to accommodate each series being represented in the game by throwing in worlds that belong to them. Someone on this site did make an interesting way of settling that, but I don't think Nintendo will go out of their way to create levels based on difficulty settings if this game is coming out in 2018. If they brought back Break the Targets and Board the Platforms, that would be cool. But again, making those like how it was in Melee would be too much. I'm not hyped for this game, but I think when we start to see more, I will be.