There are plenty of places to expand on the original story that would make good side-stories for a campaign. For example, my brother is currently working on a side-story for the Nerazim following Vorazun that will lay out how the Nerazim survived on their own. Here's a list of ideas to get you going.
Project Shadowblade: Expand on why Gabriel Tosh hates the Dominion.
Stealing the Hyperion: Illustrate events leading up to the Raiders stealing the Dominion Flagship, Hyperion.
Infested: The Story of Stukov: Explain what he tried to do to exact revenge on Emil Narud.
The Adept: The Adept named Talis likely had a role to play before she died in the LotV prologue. Tell players what it was.
The First Ascendant: Describe the history of Alarak and the efforts he took to advance Amon's influence until he discovered Amon's betrayal and Mal'lash's web of lies.
These are just a few, but there are many more storylines to choose, and new ones to make up that are parallel to the original timeline.
If you're going to create a campaign and want some help I suggest at least presenting a thought out idea or creative direction rather than a completely open ended "what should I make guys?"
Yeah, I could have put a little more of that (or any at all really)
It takes place in an alternate universe where Kerrigan was killed on Char during the assault at the end of Wings of Liberty, Raynor suicidaly attacks Augustgrad and kills Mengsk while the main Dominion army is on Char. Artanis begins to reclaim Aiur and during the attack the Protoss' Connection to the Khala is shattered, leaving them in a second Aeon of Strife. The Tal'darim and Nerazim are now the most united Protoss factions.
The Terran Campaign will be the first one I plan to do.
The United People's Protectorate is a Terran group that came into power to fill the Vacuum left by the Dominion. The Protectorate seeks to eradicate the Protoss and Zerg, Believing that doing so will ensure Humanity's security. The Protectorate has passed a law permitting the imprisonment and/or execution of people labeled "Dissenters", Anyone who speaks out against the Protectorate. A former Dominion General, Della Starla, leads a group of people labeled dissenters and forms a rebellion seeking to take the Protectorate out of power.
The Terran campaign is my most specific plan at the moment, the Zerg will have a new swarm leader (possibly Stukov) but I'm unsure of what the swarm's goal should be. (Does he adopt the Overmind's plan to destroy the Protoss?) The Protoss campaign will deal with an Executor who seeks to reunite the Protoss without the Khala.
The main problem I have with my current plan for the Terran is that I feel it's too similar to the Raiders Vs. Dominion story and would like some advice on how to differentiate it.
Some ideas I had on the Gameplay side,
Nerfed Siege tanks but increased Terran's other Defensive Capabilities, allowing the construction of Sandbags with a low cost and about 250 health and the Perdition Turret from the campaign and the Devastation Turret from Swann's Co-op missions.
Allowing the Zerg to research a strain from the building that enables the unit (I.E. The spawning pool contains upgrades for the Raptor or Swarmling Strain. Only one can be chosen) I feel like this will emphasize the Zerg's adaptability.
An overall drastic increase in the amount of upgrades.
If you're picking something as a direct alternate timeline to the original one then you've a got a whole lot of writing to do with hybrids and Amon, not to mention killing off a lot of characters far before the events of HotS even take place. Also if I may ask, why use the United People's Protectorste when you can just use the UED? Bring back Admiral DuGalle and his fleet of UED sent to replace the crumbled Dominion government. It actually fills a lot of plot roles under the assumption they still can recreate a new Overmind who would be able to resurrect the QoB and continue the Infinite Cycle? If the Infinite Cycle is broken then everyone must be fucked up at the end because otherwise you would have to throw established storyline with Amon out the window.
Like deltronlive said, start small. Your plot seems really complicated, so you'll need to piece everything together somehow or there will be that one person who will make a big deal of it. I don't know if you saw my brother's suggestions for starter campaigns. A single-storyline campaign of 5-10 missions is never too small.
Not entirely on topic but I've never understood how everyone is fucked if Kerrigan dies, Supposedly the Keystone absorbed the energy Kerrigan held as the Queen of blades and Narud used that to revive Amon, so if she dies before she gets deinfested, No energy for Amon to be resurrected right?
Back on topic, I'm open to the idea of the UED having taken over instead, none of the pre-campaign is set in stone. The UED might in fact fit better, as I have toyed with the idea of the Protectorate using UED Overmind research to control Zerg Broods. Any way things go though Mengsk dies, Cause fuck Mengsk. :P
I do like the idea of DuGalle facing off against infested Stukov, knowing he caused that to happen to him. The reason I was aiming to kill off pretty much everyone is so that I could start anew while keeping Broodwar and WOL as history basically and I didn't like Amon as a villain very much, so I tried to cut him out. My scope perhaps was a bit large, but that's what I came here for
What smaller aspects of the game would you like expansion upon? (Ex. Dark Protoss.)
If u mean by races than alarak story is very intriguing. Or dehaka,stukov even the U.E.D
What type of maps do you enjoy most? (Ex. Destroy the enemy base, Fixed Force Missions)
I find when i play any game is when it offers variety. I cant speak for all but i think many people agree having one style of campaign can become tedious. For instance a whole campaign with "kill every unit/building" its fun a few missions but get tedious after a while. Blizzard for example i adore their campaigns but almost every mission is a timer mission.... where u rush rush rush. So having a mix defend/destroy/hold out campaigns is always the most fun for me.
What mechanics would you like to see? (Ex. The ability to infest a command center)
Adding mechanics is really what makes blizzards campaigns great. Like rising lava or supernova. Would be great to see more of that.
What campaign and melee units do you want/not want? (Ex. Add Firebats, Remove Hellions.)
Its always fun to add units that are either underused or have been scrapped from the editor like the shredder and co.
How can bad units be improved? (Ex. Lower cost for Wraiths)
Yeah the idea for improving bad units is cool. I find the scout in sc1 is really to expensive and i almost never used it.
Regarding the maps, I think most people agree, I've been watching Jayborino's playthrough of Insurrection (Great youtuber as I'm sure you know :P) and his biggest complaint seems to be that 90% of the missions are destroy everything. At the same time, I personally felt that starcraft 2 had way too many "Get to this point fast or you lose!".
A concern I have with adding all the units in, is that, for example the Firebat and Hellion's roles overlap quite a bit and I only ever saw people using one or the other in WOL.
I think there are several units that could use some buffs, (Diamondback, Wraith, PREDATOR, Corpser Strain Roaches, Creeper Strain Swarm Hosts, Noxius Ultralisks to name some I thought of off the top of my head) But I also want to know what other units you think should be fixed.
The Fbat and Hellion don't quite overlap in my opinion because people virtually never use the Hellion, even in the mission Outbreak. Fbats can just get healed up and they have +2 armor upgrades so they're always the better option. They are very useful in Zerg missions whereas Hellion's slow rate of fire and straight-line AoE makes crowd control difficult.
To be honest, the units don't really need buffs or that many changes at all. Custom campaigns aren't always about the units as much as they are the gameplay mechanics and mission quality. In a custom campaign, it's better to make the player's race a subfaction of another if you want to change the units within the data editor. It keeps things simple and avoids making things too complicated for a player.
Their roles don't really overlap, it's just that the missions don't lend themselves to making Hellions. They were made as a harassment unit in multiplayer - fast, cheap, deals high damage to light units like drones and zerglings - get in, get kills, get out. That isn't really applicable to the campaign since bases are always gonna be fairly well defended and attack waves are always going to be reasonably large. If more missions were like the Reaper or Vulture missions where you're supposed to run around the map collecting things then maybe the Hellion would be used more.