I've just got back from a long hiatus and It's painful to still see Hydroid in the state that he is in. This is the last post I'm making on him then go on another long Hiatus away from the forums.
Whomever at DE reads these and makes some of these changes happen I'd like to personally thank you for what happened to Frost he's a joy now.
Hydroids Biggest Problem
Range and Radius
I'm gonna blanket statement this because it's the single biggest flaw in his kit. The Range and Radius of all his abilities is why there is no synergy with them all (Except Swarm). The BIGGEST thing you can do for him is All abilities cast in Undertow have the same Range/Radius of Undertow. This is more of a Nerf than a buff but I'm willing to bet, if you do this, his kit will have more Synergy and enemies might actually stay in Undertow.
Why:
Doesn't matter if your Tempest Barrage can hit targets in Undertow if you can hit Undertow in the first place. Tempest Barrage has a fixed Radius of 10m and Maxed out Undertow has a range of 6.69m. Yes, the blast range can still cause enemies in undertow to be hit BUT terrain prevents that from happening.
Tidal Surge being 2x the size of Undertow is the primary reason why enemies won't stay in Undertow. If you spam Tidal Surge enemies stay in Undertow but as soon as you stop they get spat out. That's because while casting Tidal Surge, Undertow's Radius becomes the range of Tidal Surge. When you stop casting its shrinks back to its original size and that is why enemies don't stay in. Tidal Surge has a Max Range of 16.8m while Undertow has 6m.I was wrong enemies in undertow cannot technically be moved by Tidal Surge and return to their origin postitions upon leaving Undertow or if you move while in Undertow. This is officially the longest standing Warframe bug in the game that I know of.
Tentacle Swarm is the only ability Hydroid has that has any actual Synergy. Why? Because Tentacle Swarm's Radius becomes the same Radius as Undertow and gets a 200% increase in damage.
So in conclusion, for all of Hydroid's Abilities sake, change Undertow to the following.
"Become a water trap and drown unsuspecting enemies. All of Hydroid's abilities Cast in Undertow have reduced range but greatly Increased power."
Power increase of (Whatever Number you deem fit) for all his abilities in Undertow (Excluding Undertow) is completely fair and not game-breaking in any way. You would literally be reducing the range of all your abilities to a Maximum range of 7 meters. This also makes sense thematically too! Hydroid should be stronger while underwater than he is on land (He's based on Davey Jones Anyways). I'm willing to also say that all Hold to casting should Increase his range and energy cost but decrease abilities damage. What is wrong with Increasing the range of Tempest Barrage to a Radius of 20m or even 40m? You can't hit anything reliable anyways. Tentacle swarm is limited to only being able to hit 20 enemies. You can't hold to cast Tidal Surge or Undertow.
Tweaking his Other Abilities
Passive:
I honestly do not care. I don't know when or why Warframes started getting Novels for passive but in my honest opinion. Hydroids Passive should be Locker and give him 2x resources and loot from Lockers. Haha, Davey Jones Locker! Common DE that's perfect for him! Then again I am from a time when Warframes did not have passives at all and if you address Hydroids Passive you need to Address Frost's, Sayrn's, Loki's, and Mag's. They all need some love and the funny thing is they are all some of the Original Warframes. Also, none of the original frames got signature weapons!
Tempest Barrage:
Leave it alone. Apart from what was mentioned above it's fine as it is. It's one of the few abilities in the game that affects Nullifier's bubbles so asking for anything more seems too much. The Augment though I'd personally like it to not increase damage at all and instead increase Status duration of the Corrosive Proc's. The numbers are up to DE on that.
Tidal Surge:
Again Apart from what is mentioned above it doesn't need a change. Maybe Holding cast would slowly make it grow or some other hold-to-cast function. The utility it provides is enough. Don't you dare touch Tidal Surge's Augment it's literally the BEST thing he has going for him currently. Maybe make the ability cost 20 energy without the augment? Damage does not matter at all and According to the wiki it's supposed to do Slash Damage. I've never seen that happen.
Undertow:
It's the biggest problem but actually not at the same time. As an ability by itself, it's one of the more Over Powered in the game. Complete Invulnerability is pretty insane. Again I would touch not touch augment it actually is extremely useful in Sorties (Agent in Defence) and Eidolon hunts (Lures and Volts) for healing things other than yourself. I've seen a lot of comments saying it should heal Hydroid by default like Wukong's ability and no it shouldn't.
Tentacle Swarm:
No change is needed either. I kinda wish impaled enemies would take a percentage damage to HP but Nah.
Hmm, things cost money so... If you need money to rework him... Reach out and I'll design and make a Delux Level skin to help fund the rework. Attachments, Syandana, and everything.
Dump pointless rant:
I'm not a fan of completely changing Warframes. Hydroid is one of those frames that I always wanted to love but he was buggy, is still buggy, and with his track record will be buggy for at least another few years. I could sit here and pump out limitless Pirate-themed frames because there is so much lore behind them. I was actually kinda bummed playing the Railjack story and seeing Sevagoth at the end instead of another Pirate or Captain-themed frame. Blackbeard, Avery, Flint, Calico, Anne, Worley, Ward, Bart, Bartholomew, Ching, and Barbossa to name a few were all truly unique. If I was able I'd make three more frames based on the myths and Legends of those famous Pirates.
Zheng the Mad Siren for whom the Bell Tolls. Based on Zheng Yi Sao (Ching Shih) the Famous Chinese Pirate of the south sea. She would ring the bells out a sea normally used in ports before there were lighthouses. Luring Traders, Sailors, and other Pirates to their demise leading her to raise the largest Pirate fleet. People argue about who was more successful Ching or BlackBeard. I don't know if it was true but it is a story about her that was passed down in my family (I'm related to Worley).
A stealth arsonist's frame based on Anne Bonny and Mary Read (Literally Called the Cross Dressing Pirate at one point) would be unique and Deadly. Instead of using stealth, she would disguise herself as an enemy to sneak around (Skin Walker). Would Literally be turning Khal and Viso into a Warframe. DE, please!
Finally, a very absurd frame based on Sete Bonet and Blackbeard. Horribly Wounded with fearsome ferocity. Warframe is based on all the cruel absurdities of Pirates. Keelhauling, Hanging, Bribery, and Gambling. Tieing up enemies and dragging them around with him or tieing them to each other or Objects (The ceiling would be the hanging part). Gambling by having an ability with 50/50 chance to kill an enemy he is dragging or sacrificing his own life/Shield/armor/Energy for a random buff or loot(if the Enemy is killed). Let him be able to buff enemies too just for pure chaos. Bribery would remove all his and his enemies' buffs and debuffs.
I dunno maybe one day I'll flesh out the concepts and post them in the forums.