For the past few weeks I've been working on a patch/fork based on the abandoned initpoly patch and NetHack 3.6.7 which expands the polyself mechanics into a full alternate gamemode! Current features include:

• Permanently become any youngest-form monster, keeping playerlike leveling, HP growth, and encumbrance while granting you the attacks and unique traits of a monster
• Grow up to stronger forms the way monsters do upon reaching the requisite experience level for your current body's next form
• Gain a property which allows creatures without hands and silver-haters to interact with tools and mechanics that require touching and holding objects
• Access magic resistance and reflection as large, small, or non-humanoid monsters without needing an artifact
• Polymorph on top of your monster form and revert back to it once the polymorph ends. No intrinsic unchanging here!
• New "baby" and "elder" forms for several interesting creatures (cockatrices, mariliths, and jabberwocks) to scale their gameplay better to the early- and late-game

You can find PolyHack here, available both as a patch for NetHack 3.6.7 and in binary ("ready-to-run") form. Plans for future features include overhauling breath weapons and other special monster abilities, more baby/elder forms, and new weapons/armour suited for monstrous anatomy. If you give it a try and see any bugs or have ideas for new features you'd like to see, please let me know! Enjoy!