Have you thought about Heroes' Bane?

Kyln I did previously have 2 Heroes' Bane in the deck, I ended up taking it out because Kalonian Hydra is just a much better 5 drop Hydra, between doubling the +1/+1 counters on ALL my creatures and having trample, it just wasn't a contest. I did run both for a little while, but found I wanted more lower cost hydras so I took them out.

Aspect of Hydra has also won games for me out of no where and fits your theme well!

Has anyone suggested Vigor ? And I like the idea, +1!

Awesome deck! If you want +1/+1 counters I have an entire commander deck here: Mutation Station (Commander). It has blue, and is a different format, but I bet you could find some cards you might be interested in using. Also, if you plan to use Corpsejack Menace a lot, it makes an infinity combo with two Scrounging Bandars

Hardened Scales would be a good addition. It does a similar thing to Corpsejack but it adds 1 extra counter instead of doubling it, but it's just a 1CMC enchantment in green. Plus you can stack the abilities so that if you have both out you can add the extra counter from HS and THEN trigger the CJM, so you double the ability of HS.
Other cards I would consider are Avatar of the Resolute and Winding Constrictor (but take it out against infect).

Hardened Scales is begging to be in this list. Same with Winding Constrictor, i totally agree with Jakot. You can also consider Doubling Season or Scavenging Ooze

Reyhan, Last of the Abzan and Winding Constrictor definitely help the deck but it pulls you away from the hydra theme and throws you more in to the +1/+1 counters theme. And definitely agree with the above suggestions if you pursue that over the hydras.

I ran a standard deck like this a while back with Predator Ooze and Blessings of Nature. Casting Blessings with Corpsejack Menace on the field can pretty much mean gg, and I found Predator Ooze to be a great fit as well. Cool deck! Got an upvote from me.

I have a very similar deck however I use the Varolz, the Scar-Stripped and Death's Shadow combo to make +26/+26 counters on turn 3-4. Also Walking Ballista would probably be better than Hangarback Walker.

Putrefy would be something id add instead of Abrupt Decay. It targets anything. Hardened Scales is also another, though already said, a note-worthy card to help benifit this deck. I understand why Doubling Season isnt in due to its price.

Llanowar Reborn for that free +1/+1 counter my dude.