Initial impressions:
The linear gameplay seems to indicate that the growth of the character will be fully directed and up to the designers; as in, upgrades, growth/transformation will be entirely linear, and doled out in accordance with the level design. Upgrades and powerups are mysteriously "well placed", like you get a specific upgrade or power up going into a bossfight, and it turns out that it's exactly what you needed to win. It's basically the same thing as in FPS games when you get a bunch of ammo and weapons just before you enter a bossroom.
That takes all the fun out of this type of game, as you'll merely be following a path that gives you upgrades to overcome obstacles and challenges. The game might allow a tiny bit of freedom through optional items, doing well, so that you may, for example, grow 1% larger or smaller than the average player, but that's almost irrelevant. It's the equivalent of an animal pushing a button to get food (Rewarded.).
In order for such a game to be enjoyable, it needs to be all about the challenge, like how the FPS games of old with this type of design worked. But it doesn't seem like that's a priority at all in this case.
This type of design just doesn't feel right or enjoyable in a linear setting. It would fit very well in a sandbox type of design, though -- as in, make it all about the creativity and freedom.
It is indeed somewhat similar to Katamari, but without the freedom and, seemingly, what made that game unique and enjoyable.
The core game design of this game seems to be basically like this; You roll down a hallway, face a challenge, get an upgrade, roll further down the hallway, face a slightly harder challenge that takes your new upgrade into consideration, get another upgrade, roll further down the hallway, face another challenge that takes both of your upgrades into consideration, and so forth. A lot of games are like that, but have additional stuff going on that obscures that linearity.
