(Eternal) Transformation feat + Human = just about any creature you can think of. Single best General talent in the game, not for power, but for granting you every single creature as a playable race, without breaking the game, enforcing dumpster diving, nor taking a ship ton of time. (And the Eternal version does nothing but commit to the bit, and say "no, you don't actually have a human form to fall back on.)

Want a Dragon? Dragon Transformation (I like the term Genotype used in 5e version, which I'll just shorten to Gene). Obviously. You get the basic shape, and abilities associated with your form, like breath weapon, that scales on character level.
Although you'd have to wait until you get Improved Transformation to give it gliding / flight. You're just a hatchling for now.

Giant Spiders, Snakes and Dogs? Obviously, they have their own specific genes.
Mimics? Similarly have an Object gene.
Beholders or Gelatinous Ooze or Black Sludge, or Living Freaking Fire? Not a problem.
And this isn't even being particularly creative with any of them.

Now, for specific abilities, or certain capabilities, the simple Transformation feat will not grant you it.

So, if you are a Werewolf, or Troll, you will still have to get access to the Life sphere with the Regenerate drawback to get your fast healing / regeneration. (But you can be big, green, and, and take Giant's Fortitude trait for a regenerating pool of temporary hit points, with your Anthropomorphic Gene.)
Basilisks would want to invest in Destruction sphere with Crystal Blast (covers the target in rock, leaving them immobile, until they break free). Not nearly as lethal as a default basilisk's instant petrification on a failed save, but usable in a normal party.
And if you want your Psicrystal to be mobile, you probably want to take the Wraith class with the Matagot archetype. (Honestly, a reasonable DM would just let you reflavor Matagot by default, But if they don't, Transformation is still an option available to you.) (Also, Elemental Transformation doesn't remove your ability to move. But I always have to throw out "You get to be a familiar," whenever I can gush about it.)

Can you be a Ham and Cheese Sandwich? No. But you can be much more evil and disgusting as a Soy-substitute Ham and Cheese Sandwich. (Take Death sphere.)
You can all be household appliances, and go on an adventure, like in that movie... from the 90s I think.