Am programmer (but I don’t work on The Bibites) and you’ve got a decent idea of how easy these things would be to program relative to each other. I’ll just give my notes on the first three suggestions though, to make sure this isn’t too long.
Adding a tool that lets you click on food to remove it, or click on the floor to add food, would be extremely easy. Would probably just involve adding a button to the GUI (and/or a keybind) which causes the mouse to delete any food objects if present, or create food if not present.
Walls would be a bit more complicated than that, but only a bit - adding a tool that lets you click on one point, then on another, and create a wall. If you wanted to make it really flexible, you could have it accept any arbitrary amount of points so long as there are at least two. You would then have code to create an object between each pair of points that collides with Bibites and prevents plant spawning.
Temperature would likely require implementing a procedural map generator, since right now the map is basically the same each generation save for randomized plant spawning.