Is this Sentient Vine Cloak magic item balanced?

Question

I've got a level 8 halfling Circle of the Moon Druid who really enjoys using quirky Wild Shapes at every opportunity she can get. I want to encourage the creative beast choices during combat, and give an alternative to the usual Ankylosaurus → Elephant → Triceratops spam that tends to happen as they get into higher levels.

My approach to a solution for this is a sentient vine cloak that would give an AC boost to Large or smaller creatures (which would work with the Wild Shapes she chooses) and a grasping vine perk that can be cast while in Wild Shape. Does this make the large or smaller Wild Shape beasts competitive vs. huge beasts while staying balanced for other situations?

Chlo'ifeltatoldos, 'Chloe'
sentient magical cloak, requires attunement

Concept and Description:
While this cloak appears as a simple vine bush when not worn, is actually an old and lost fragment of the Fey Wild. If able to prove themselves capable, Chloe might be convinced to travel with and protect an individual. The vines wrap themselves around the bearer, adding a layer of protection as far as it can stretch. It will even lash out if it's in trouble... or if it's angry... or if it's bored...

Features:
+1 AC to any large or smaller creature.
Chloe can cast Grasping Vine at 4th level, centered on itself with a save DC equal to 10 + your Proficiency Modifier. Chloe makes concentration saving throws for this using your Proficiency Modifier. Once this feature is used, it can't be used again until finishing a short or long rest.

A few notes:

• This would technically still allow the Grasping Vine ability while in a huge beast form (or any other time).
• This could create a semi-double concentration situation, as Chloe could be concentrating on the Grasping Vine while the bearer could concentrate on something else.
• The concentration check that Chloe makes is whenever the bearer is damaged. So there could be a concentration roll for the bearer, and one concentration roll for Chloe.

Answer 1

It's probably not overpowered, but it might not achieve what you want

In terms of item rarity (as a measure of power) it lands at about rare (+1 to AC and cast a 4-th level spell once per SR) possibly bordering very rare. However, only an AC bonus is not enough to bias smaller Wild Shape forms from Huge ones, and it is probably not enough to shift a tactically minded druid player away from those forms.

The main reason the Huge creature form "spam" occurs, it that high CR (>3) beasts are few and Huge. A DnD Beyond search gives that the only swarm of cranium rats has a CR above 3 and size smaller than Huge, and that the options in general are limited.

Unfortunately a lower CR beast with a +1 bonus to AC is probably not enough. Following the guidelines in the DMG for Creating a Monster (p. 273) such a small change doesn't even lift its defensive CR let alone it's total one.

There are some options though; if you only would like your player to have access to more (and smaller) options for combat; you could either let some monstrosities (hippogriff, owlbear, etc.) or other creature types (pegasus, wyvern) count as beasts (at least for Wild Shape (this might get them access to stronger effects than grapple and prone as beasts are limited to though)), or you could brew up some new (and stronger) beasts. As beasts are fairly limited in the abilities they use, the guidelines in the DMG (p. 273) hold up fairly well. This could even be played as a subtle reward during Exploration as meeting strange new beasts can be almost as rewarding as a magic item (at least for the Druid). You could probably find home-brewed beasts on DnD Beyond, the DM's Guild, or a number of other places.

If you would rather focus on the creative side, adding elements to the environment (both during encounters and outside them) which a particular beast form (because of its different movement modes, size or other features) can excel, can make Wild Shape much more rewarding all round. This approach does require a lot of creativity on both player and DM though. (As a general note I would advice against planning a lot of such interactions for the player as you will likely only disappoint yourself that they didn't find them, rather just include more interesting details to geometries and details to your environment and let them play.

If you still would like to give a buff to beast forms though you could look at giving a bonus to the DC of the beasts abilities (with a similar limit as you've used for AC), as making those effects more reliable (A DC 11 save is quite easy for a lot of big and scary things) will also reward using more "quirky" beast forms.