Timberwolves is actually a really good idea, and something I could easily see myself using. The original idea was to use those haunted tree things from the S1 opener that Pinkie laughs at, but Timberwolves would definitely work.
Something I'm considering/have decided on is to have the difficulty level increase as the player goes deeper into the forest (because that works thematically, it matches expectations, it stops the forest from being one homogeneous blob, and it creates a super-straightforward early goal - go deeper). So I might use the haunted trees for the outer reaches, then the timberwolf idea for the inner portions. Hmm... Definitely something to think about.
The node-based maze is definitely something to consider, too. I'll keep that in mind. Thanks!
What you could also do is everytime you place a tree, check to see if it forms an L shape out of 3 trees. If it does then don't place the tree. and assuming you can move diagonally, that shouldn't block any paths
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This one I'm not the biggest fan of. I'm worried that removing all those L shapes is going to limit what type of layouts can appear way too much - it's basically saying "I only want lines of trees in the x or y direction, possibly with spaces between them, and one line going in the x direction can never meet with a line going in the y direction".