Joo Dee, One of ManyJoo Dee, One of Many gives you a way to add more tokens to the board, sculpt the top of your library, fill your graveyard, and sacrifice some creatures or artifacts - all on a two-mana body
There arent really any other commanders that make sense for hydra commanders, although +1/+1 commanders and X-spell commanders can sometimes make use of hydras
At the beginning of your upkeep you reveal cards from the top of your library until you reveal a creature card
Yuna is a very open ended commander with more potential than there are slots in the 99
If your deck is trying to cast as many X spells as possible every turn, and wants X to be as much as possible, we probably want to prioritize ramp more highly than just 8 or 10 spells - in fact, I can see running 60 mana sources, like this Vaevictis Asmadi, the Dire deck