I'm not entirely sure since they hinted in an interview you kinda "need" more players for some parts of the storyline so that could hint that it will be played online in some form. I'm positive however that there will be an offline mode but I don't think it's been confirmed yet.
What I actually hope for, and maybe some mods can make it happen, is a world wall that wraps around to the opposite side of the world. If you could make a Hytale world "round" by warping players from one side to the other, you could have a set size for your world, but also allow players to get home. Thoughts?