This app was mentioned in 13 comments, with an average of 1.23 upvotes
Linear Quest is fantastic and really well executed: https://play.google.com/store/apps/details?id=com.llgroup.linearquest
Crusaders Quest is well executed as well! https://play.google.com/store/apps/details?id=com.nhnent.SKQUEST
I have played long time ago this game: https://play.google.com/store/apps/details?id=com.llgroup.linearquest&hl=en_US
Probably the most type of non-railroady incremental games might be RPG type games that maybe let you choose a type of class, but even then, that completely depends on how the game plays out. Generally with most incremental games, choices in the terms upgrades with a path are either boosts to idle gains, or boosts to active gains. Or you can sort of split down the middle. Clicker Heroes is a good example of this.
I think the least linear game for play style I've seen is... ironically, a mobile game called "Linear Quest".
Yeah, I know.
It's a small RPG game with a lot of skills, weapon types that matter, and you can put stat points in a choice of stats however you like, making your character quite unique. And on top of all that, you are able to pick a class after a certain point. It also apparently has multiplayer, PvP and a "hardcore mode", although I haven't tried any of these yet.
Maybe because it doesn't have any real idle elements though, is why it at least feels a bit less rail-roady? A lot of incremental games in my opinion seem to focus more on the idle/clicking aspect rather than the actual incremental aspect, if that makes sense. Rather than focusing on the magic formula that makes X number go up faster in the shortest amount of time or in the least amount of effort, Linear Quest seems to be more on how you want to get X number to go up.
Of course you can argue that everything ultimately is made to make your character progress to the right more and more; but the ability to seemingly have different builds with ultimately vastly playstyles with other people in a party so you can all go to the right seems to shake up things a lot from the usual formula. For example, you could make a full on support character, relying specifically on others. And while eventually you can unlock all the skills (it would take a very long time), you stat builds and classes will still be unique enough for how you play your character.
Since I talked about it, here's the game I guess in case it piqued anyone's interest. But again, if you're looking for an incremental game with idle mechanics, this one isn't it:
Normal: https://play.google.com/store/apps/details?id=com.llgroup.linearquest&hl=en
Hardcore: https://play.google.com/store/apps/details?id=com.llgroup.linearquesthc&hl=en In hardcore mode, if your character dies, a score is submitted to the leaderboards and your character is deleted. In Hardcore Ultra, your save file is simply not save - the character is gone once you quit the game or die.