Lowest damage potential in the game. He needs damage buffing or armour removal
Tentacles move enemies out of range, necessitating radial weapons (and making most of them significantly worse due to the length of the tentacles)
THE CAST TIMES
Also a mild nuisance that Barrage essentially doesn't scale off range. The knockdown radius does but big whoop, that's not how you use it anyway.
I love Hydroid, and I have a hot take that he's really good for when you get him due to having what you need to clear the starchart in a really fun way, but by god the scaling on this poor frame is atrociously bad.
Anyway, some hopes:
Passive is some kind of damage buff or a loot buff.
Undertow and Tidal Surge are merged. New ability is a damage vulnerability or an armour strip. (I don't see them buffing Barrage given they completely passed over Corrosive Barrage previously, maybe for Helminth reasons.)
NO MORE CAST TIMES
Tentacles have damage transference a la Tornadoes, or at least don't lift enemies up in the air where you can't hit them. Suction effect so enemies actually get caught in them.
Tentacles scale off range or power strength, or at least spawn intelligently according to the navmesh so you don't get random wasted tentacles doing SFA.
I'm of the opinion that reworking warframes generally is not worth the effort but there are exceptions, hydroid being one of them. In most cases I think adding a new warframe is better than a rework in case there are enough people that liked the old abilities. With that in mind I think his abilities need to be improved and made more fun instead of scraped.
reduce cast time and remove charging. He's just too slow.
replace puddle with a water form that starts at 100% damage reduction and reduces to 0% damage reduction towards the end of the ability as he dries out and as it negates damage.
increase damage for his abilities by the number of tentacles spawned. Damage of abilities increases by 10% per tentacle active.
tentacles don't grab enemies but attack them and stun them. Tentacles grab corpses and throw them at other enemies.
killing an enemy has a chance of spawning a tentacle in addition to ground slams. Raise the maximum number of tentacles from the passive to 5.
enemies hit by tentacles take 10% more damage from all sources every time they are hit.