Replayability IS mandatory in a game like this. The player has to wonder what would happen if they used a different character or approached situations in different order, or picked different sides.
Reconciling a linear story arc AND "choice&consequence" is difficult. It is a learning experience in a category of its own. A number of issues have to be overcome, including these:
1) Controlling the player's progression, but loosely, without "hovering over the shoulder" or creating invisible walls in the way other games do.
2) Not having the main arc overshadow the secondary quests and make them seem small in comparison.
3) Not making secondary quests be immediately recognized as secondary quests.
4) Linking secondary quests into the main plot arc without destroying the arc.
5) Allowing for a more-than-superficial breathing room within just the main arc itself.
6) Not making the game easily breakable/unfinisheable due to player doing unforeseen things.
7) Not making the game in a way where player simply avoids certain situations the next time they start over. This makes "major twist endings" a difficult affair, but perhaps your key party NPC's shocking but inevitable betrayal the kind of thing better left for another medium, such as film.
I'm happy to report that the issues in the above list have been, for the most part, resolved. Shelter's design is inching forward with steadiness of a Soviet bulldozer and cleverness of a Soviet bulldozer.
Wait-..
