Alright, I think I have something viable for a revised Polymorph spell. It maintains a lot of the options available via the Monster Manual. The spell is a bit longer and more complex than most spell write-ups, but it has the benefit of being self-contained and not requiring anything outside of the spell description. I know it doesn't add some of the bells and whistles of some forms such as Stomp, Blood Frenzy, Web, ect, nor does mine come with the ridiculous number of hit points of the original. But I tried to address this by slightly increasing damage of the Battle Form, and through the additional utility of the Stealth Form. The spell is also able to be upcast.
Polymorph
4th-level transmutation
Casting Time: 1 action
Range: 60 feet
Components: V, S, M (a caterpillar cocoon)
Duration: Concentration, up to 1 hour
This spell transforms a creature that you can see within range into a new form. An unwilling creature must make a Wisdom saving throw to avoid the effect. A shapechanger automatically succeeds on this saving throw.
When you cast this spell, you decide whether the target take on a
Battle Form,
Stealth Form, or
Baleful Form, each of which is described below. You can determine the physical appearance of the form taken. The transformation lasts for the duration, or until the target drops to 0 hit points or dies. While in this form the target can’t speak, cast spells, or take any other action that requires hands, speech, or activates equipment.
Battle Form. While transformed, the target gains the following benefits:
• The target gains 25 temporary hit points. If any of these remain when the spell ends, they are lost.
• The target grows to Large or Huge size, unless they are already larger.
• The target’s movement speed increases to 40 ft, unless they already have a faster movement speed.
• The target loses any additional movement speeds. Instead they can choose one of the following secondary movement speeds: climb 40 ft, or a swim 60 ft. If the target is granted a swim speed, they can breathe both air and water.
• The target loses their normal attack options. Instead, the target gains a melee attack and a ranged attack. When the target takes the Attack action, they can either make up to 2 melee attacks, or a single ranged attack. Both the melee and ranged attack uses your spell attack bonus. The melee attack has a 10 ft. reach and deals 3d12 + your spellcasting modifier. The ranged attack has a range of 50/100 ft., and deals 8d6 + your spellcasting modifier. When you first cast this spell you choose whether the attacks deal bludgeoning, piercing, or slashing damage.
• When the target takes the Attack action, they can make two attacks instead of one.
Stealth Form. While transformed, the target gains the following benefits:
• The target shrinks to Small or Tiny size, unless they are already smaller.
• The target’s movement speed increases to 60 ft., unless they already have a faster movement speed.
• The target loses any additional movement speeds. Instead they can choose one of the following secondary movement speeds: climb 40 ft, or a swim 60 ft., fly 60 ft., or burrow 15 ft. If the target is granted a swim speed, they can breathe both air and water.
• The target gains darkvision 60 ft. unless their darkvision is better, as well as blindsight 60ft. The target’s blindsight does not function while deafened.
• The target gains a +10 to Perception and Stealth checks.
• The target cannot take the Attack action.
Baleful Form. While transformed, the target changes in the following ways:
• The target shrinks to Tiny size, unless they are already smaller.
• The target’s speed is reduced to 10 ft., and loses any secondary movement modes.
• The target cannot take the Attack action.
• If the target takes damage, they can attempt a new Wisdom saving throw with advantage, ending the spell on a success.
At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the temporary hit points granted by the battle form increases by 10 and the damage dealt by the battle form increases by one damage die for every two slot levels above the 4th.