Actually, might I start a discussion with you on the matter of linearity? I'd like to pick your brain on the subject, since you're the first person I've received that comment from (I've tested it with some close friends already).
The game does open up a little bit after the first chapter, though I suspect not to the degree that you might like. I've mostly allowed the player to backtrack to previous areas, and I intend to include optional areas as well (secret dungeons, bonus bosses, and the like). However, that kind of content isn't in the game yet. Basically, what I'd like to know is this: do you think the Game is too linear, or do you think the Story is too linear? I'd like the player to feel like they have agency, and knowing the answer to that would help me address that issue.