Forests may look like a lot, but they're actually pretty boring. Have you ever tried taking a hike through a forest in survival? You have no idea where you are, and all you can see is trees, going for kilometers of flat land. So why not add something a little unique to forests? The forest altar would be a simple little structure to give the forests more variety. They would be rare enough to not be common, but common enough to be found with some exploring. The structure would be pretty small, about 4x4 blocks in total, and would be made out of mossy cobblestone. The middle of the altar would be a pedestal with a chest, which could include a range of items, including iron tools or gear, wheat, cocoa beans, and maybe the rare enchanted book.
The three altar types:
Of course, why stop there? Three different altars makes this way more interesting.
These are the oldest of all altars, and are made of mossy cobble. Some blocks may be missing, due to the structure crumbling. Usually contains boring items, such as wheat, or music discs, but can also contain enchanted books.
These altars were made for worshipping evil purposes. There is a mob spawner below the chest. Loot is usually a little more on the rare side.
These altars are in shiny new condition, made out of stone brick, and not a piece missing, but they are the rarest of the three. They usually contain good loot, ranging from an enchanted book to iron tools.
New structure? Why not, but I think there should be a bit more than just a chest. I'm not saying a temple, but just walking in, getting the chest, and leaving doesn't really make it all that interesting.
Maybe they're built around a larger tree (2x2 or 3x3 trunk), and the tree is a new tree called the 'Ancient Oak' that has a more saturated and older look and it harder to break, and the chest is inside of it?
That was thought up in about 30 second, so it's probably not of my highest quality, but other than that, I can't think of much else.
Maybe the tree is only the very top with the branches, and the tree actually extends into a dungeon-like area underground that has like silverfish or spider spawners, and 1-2 chests are mixed in with the roots of the tree, and you have to stop the spawners and dig into the tree to get to the chests.
Again idk, these are off the top of my head ideas