The Awaken Spell Blows My Mind

 Awaken is a 5th level transmutation spell available to Bards and Druids. It has a casting time of 8 hours, a range of touch, and consumes a 1000-gold agate as its material component. You can cast this spell on a beast or plant with an intelligence of 3 or lower. When the spell goes off, the creature gains an intelligence of 10 and gains the ability to speak one language you know. If the target is a plant, it also gains the ability to move its limbs, roots, vines, creepers, and so forth, and it gains senses similar to a human's. Plants can have stat blocks similar to an Awakened Shrub or Awakened Tree, whichever is appropriate as determined by the DM. The target is charmed by you for 30 days or until you or your companions harm it. When the charm condition ends, the creature can choose whether or not it wants to remain friendly to you, based on how you treated it.

So...

This is crazy.

A 10 intelligence is meant to represent average human intelligence. That means that the creatures go from being simple animals or mindless plants to... well, people.

And that's where I feel like the potential for this spell gets really crazy.

Once you have the ability to speak and the human capacity for abstract thought, I don't see any reason why a creature of this sort couldn't take class levels. It might be slow going at first if they go with something like Wizard or Artificer (it would take them until level 19 to max out Intelligence). But, like, a Deer has a Wisdom of 14 - meaning they could be a decent Druid. Or, indeed, with a 16 Dexterity, they could go Monk - which might also allow them to deal with the fact that they'd have trouble finding armor and might need to rely on unarmed strikes (with their antlers, for instance) without the use of opposable thumbs.

Seriously, I think you could, using the custom lineage rules, actually make a player character who is an awakened beast. Unfortunately, primate beasts (such as an Ape) are actually too smart to benefit from the Awaken spell (an Ape has an Intelligence of 6, meaning that they're actually just as smart as the druid from my original campaign).

But generally, as a DM, always remember that you can easily justify talking animals and plants in your campaign.