Semi-open.
Pretty sandboxy.
Fairly replayable.
It's "Open" in that you get one big large map to start with and lots of different quests to do that in that map. Outside of the main story missions there's several kinds of optional missions to do. As you progress and more areas open up, you largely get to choose what you want to spend your time doing/going after.
The replayability ultimately comes from the hunts and fighting the monsters, and whether you enjoy them. That's the meat of the game and the experience. Fighting monsters and learning them and mastering them, then getting offered harder and harder versions of them on top of the new ones that get drizzled in over the course of gameplay.
There's several areas with completely different biomes that you can spend time running around in free roaming, gathering, exploring, discovering a few things, fighting monsters off the cuff rather than as part of a mission if you like. That stuff was fairly engaging for me too, I found it a nice way to play the game but chill, just running around in Expeditions.
In terms of sandbox where you bait monsters to fight each other and what not, I don't know if MHW really delivers on that front. Some of it has a slightly scripted feel about it that the action and overall gorgeousness of the game does a lot to conceal. But it's not the kind of sandbox experience that offers a lot of SCIENCE! opportunities. Despite the fact "Research" is a word that gets thrown around liberally in game.
I dunno. I don't get tired of fighting monsters. I enjoy getting better at fighting them, even ones from the early game that are just juiced up for the end game. There's lots to explore, and a nice deep rabbit hole of gear and upgrades to fall down that is good for hundreds of hours. You can spend an incredible chunk of time just working to get all the weapons in a given weapon class....and there are many weapon classes to choose from.
I'd really watch some gameplay. Yeah it might spoil a monster fight or two for you, but it'll give you a sense of whether doing THAT for hundreds of hours looks appealing. It's not the kind of game where I can say "Oh if you enjoyed the challenge of Dark Souls etc... you'll like this." Despite a lot of the same similarities and feel and style and what not, they hit different beats.