Showing relevant entity nbt data in the tooltip could be very useful. For example:
Owner (for tamed animals), active potion effects, armor bar, movement speed and jump height values, breeding status (actively breeding/on cooldown/ready), what the mob is aggro'd against (all players, specific playername or mob type, none), markings name (cats, horses, parrots, axolotls, tropical fish etc), pollen (bees, yes/no), stung (bees, yes/no), hive pos (bees again 🙂) and more!

Vanilla style sprites for many tags would be nice to avoid the bar growing too overwhelming. For example, a forward arrow for horse movement speed, and an upward arrow for its jump height.

Ideally each data tag could be configured such that one could choose what states would be shown (i.e. only showing when an animal is breeding or on cooldown, but omitting the tag when it's ready to breed)
Also, making these tags possible to override by a server owner to prevent what they may see as cheating could be an important addition.

This would all be intended to fit under the entity's name and health values, similar to certain block NBT data currently shown:

Honestly, I feel like this is a feature for a plugin. My current view for built-ins is that they only contain client-only components. We'll see if I changed my mind.

This could exist as an addon for WTHIT similar to megane, so that WTHIT itself would not require serverside components. Alternatively, certain features could just not function without player access to the /data command.

Of course, you could also avoid restricting such features at all, as several other mods have done.

This is type of feature may be better suited to QML's config API when that officially launches, as it would no longer require a server-side component to force-disable more subjective features.

erm... good idea, especially the potion effect

Oh well megane has completed a lot and it looks great