I know there have been some announcements for a Hydroid rework and a new ability but honestly I think most of his kit has a large potential for synergy if they just refresh his current abilities as they have done for other old frames.
Slight changes that could refresh his abilities and make him keep up with the rest of the cast without needing a new ability:
Tempest Barrage: Make it an aura that follows hydroid instead of just marking an area to barrage. It could have an energy drain as a channeled ability and could have armor/ shield reduction not based on status procs with some with some adjustments to the type of stagger and knockdown it does for cc. This could increase his survivability through pure CC and allow him to deal with heavy enemies with ground finishers. Also it would be great to not have screen shake the entire time it is activated so you can see straight.
Tidal Surge: Update the type of vacuum it has so that it is more comparable to mags buffed pull. Holding the ability should do a slower moving wave that you can steer like Revenants reave and collect enemies while a tap can give you that fast straight line dash without steering.
Undertow: I see a lot of potential in undertow if it wasn't a channeled ability. I would like to see it given some properties similar to Grendel. If you hold the ability, the puddle forms and creates a vortex of spinning water with a slight vacuum on it pulling in nearby enemies before the water calms. You could set up to 2-3 puddles in strategic spots utilizing tidal surge to group enemies into the puddle pulling either health or energy out of enemies based on how many enemies are in the puddle and what damage you are doing to them. Tempest barrage should still be able to strip armor/ shield from enemies in undertow and you should be able to shoot the puddle to add damage to the DOT.
Tentacle Swarm: I see a lot of potential for this ability if it was given properties similar to Zephyrs Tornadoes. You should be able to add damage to the tentacles by shooting them applying dmg from your weapon and increased damage to the tentacles from shooting them. If adding damage to the tentacles by shooting them isn't an option, then have tentacles scale from damage collected in Undertow as if you are "feeding the Kraken". Tentacles should be able to hold more than one enemy and have a slight vacuum applied to them (they could impale enemies holding up to 4 per tentacle). You could use the improved Tidal surge to bring enemies into Tentacle swarm for damage and finish them off by putting them in a puddle and buffing your team or you can group enemies into a puddle, buff your team, and then pull them out of the undertow and hit them with tentacles.
Here is how the synergy would work:
Cast tempest barrage to have a weak but constant cc ability up and running that debuffs armor an shields.
Start creating undertow puddles in strategic areas bringing in health/ energy. Builds that focus on health restore would be good for large health/ armor pool + rage/ hunter adrenaline builds and energy restore would be good for shield gate/ negative efficiency builds.
Cast Tentacle swarm for CC and damage in places where you can't place puddles.
Use Tidal surge to corral enemies into either the undertow or tentacles.
