I thought of this build last year, thinking of finally using it. Just wanted to see if there was anything wrong with it RAW.
I wanted a tank to protect the squishiest or most relevant character on a battle field almost having a similar play style to Yuumi from LoL (jumping to support who needs it the most)
These are my notes from when I first thought of the build:
"This works for any druid but Moon Druid will work best. Due to the ability to cast spells in wild shape and extra temp hp.
Be a druid
get the tough origin feet (optional)
At level four get the Mounted Combatant feat.
Transform into a tiny creature and mount a friend that you want to protect. Now all attack on them can be directed at you and any DEX saves they need could be fully avoided if they succeed the saving throw.
At level five you can now refresh your temp HP using spell slots to wild shape again.
If you multi class into warlock or your DM allows you to use the rune carver feat or the old version of magic initiate you can take Armor of Agathys. Now when hit, you will deal damage.
Pros of multi class warlock, you get a level one spell slots on short rest. You also get an invocation which can be used for advantage of concentrating checks, an upgraded familiar, extra spells, or a summonable weapon. At druid level 5 your extra spell slots can be used for another wild shape
Cons of multi class warlock, loss of spell slots progression, takes longer to get a flying speed. 13 Charisma needed"