Describe your idea, include images if necessary

all flying mobs:

• mining speed will be normal while in air

vex

• always be in flying mode, and can't unfly with double jump (that way you won't accidentally fall through bedrock and into the void)
• can go through portals (nether and end)

wither/wither skeleton:

• immune to wither effect

underwater mobs:

• same mining speed as normal while submerged in water (both floating and on the ground)

skeleton/stray/bogged:

• ability gives you an arrow of the type (skeleton: normal, stray: slowness, bogged: poison) with a cooldown of 600 ticks (30 seconds)

allays:

• regeneration level 2 or 3 while playing as mob, and remove hunger bar

bugs:

• mobs with a bigger hitbox than 1x2x1 will get stuck in the corner of blocks after using a portal or dying
• you will still lose hunger while playing as a monster, you can see the hunger bar is lower afterwards by transforming back into a non-monster
• playing as a ghast and using the ability while moving quickly can sometimes cause the fireball to explode inside you, possibly one shotting you
• while in the ground as a vex some chunks don't load, if you switch to spectator gamemode then all of the chunks nearby load

• mining speed will be normal while in air

implemented as of 6bedaae

• always be in flying mode, and can't unfly with double jump (that way you won't accidentally fall through bedrock and into the void)
• can go through portals (nether and end)
• while in the ground as a vex some chunks don't load, if you switch to spectator gamemode then all of the chunks nearby load

I won't implement it for now because it's caused by Minecraft's "noPhysic" attribute and I'd have to change how it works, therefore might break some other mechanics.

• immune to wither effect

Implemented as of be133a4

• same mining speed as normal while submerged in water (both floating and on the ground)

implemented as of 6bedaae

• ability gives you an arrow of the type (skeleton: normal, stray: slowness, bogged: poison) with a cooldown of 600 ticks (30 seconds)

Implemented as of 1829a65

• regeneration level 2 or 3 while playing as mob, and remove hunger bar

Implemented as of ea51bd0

• mobs with a bigger hitbox than 1x2x1 will get stuck in the corner of blocks after using a portal or dying

Sadly, this isn't really fixable for now, because the entities are like "bigger" than the portal itself.

• you will still lose hunger while playing as a monster, you can see the hunger bar is lower afterwards by transforming back into a non-monster

fixed as of 61b81d0

• playing as a ghast and using the ability while moving quickly can sometimes cause the fireball to explode inside you, possibly one shotting you

The only way to fix this would be to force the fireball to spawn a bit outside of the ghast but therefore also missing it's target.