Halo is one of my favorite series ever, but I feel odd saying that I wouldn't like Master Chief at all in Smash when everyone else seems to like him.
I have a few reasons:
1. Master Chief and Halo have both never appeared on a Nintendo system... ever. One was planned for the DS, but it never happened for one reason or another. You could reaaaally stretch it and say that the Minecraft mash-up pack counts, but.. come on. That's not MC, that's a costume. A blocky one at that. There's no reason for them to put in a character that has no association with Nintendo over characters like Banjo-Kazooie or Steve that very much do and would appeal more to the Nintendo audience, especially for a DLC character which they rely on people wanting to buy.
2. I don't think MC is the best representation for his genre compared to a much more classic, genre-defining character with a very strong history with Nintendo: The Doom Slayer AKA Doomguy.
What does/would Master Chief really do that Doomguy doesn't? The only thing I can think of that's really unique to Halo is the vehicle system and the energy sword. He likely would not be able to use vehicles in Smash, and, Doomguy even gets a cool energy blade in Eternal along with a grappling hook and blade.
It could be argued MC has a few more cartoony weapons to use in favor of Doomguy, however I think the Plasma Gun, Static Rifle, EMG, Rocket Launcher, Reaper, Gauss Canon, Lightning Gun, Grenades, other abilities, and the BFG (it has a non-swear canon full name for it if it were an issue) are enough to make a moveset out of Doomguy also emphasizes his ability to use hand to hand combat a bit more than MC in the new Doom games (even the classic has him using beserk a lot, and they use it as his Ultimate in Quake Champions.)
2.1 Also this is a very very minor thing but if they used the Halo 4 or Halo 5 design for Master Chief that would kill any and all hype I'd have for him. It's so ugly and doesn't look very similar at all to his original design made by Bungie. I know Halo Infinite is coming and makes this issue moot but they still use the 4/5 suit in a lot of new promotional material such as ads so I thought it was worth bringing up maybe probably not
This post isn't really related to any post recently, but I keep seeing Master Chief brought up a lot and just thought I'd throw my opinion into the pile since I like Halo and Smash a whole lot.