WoL campaign units, structures and upgrades that aren't in coop
I thought I'd make a list of WoL campaign units, structures and upgrades that aren't present in coop, with a view to considering which terran commanders could benefit most from these being added to their kit.
Mercenary units (except dusk wings)
These are noticeably missing from coop. I'd replace Raynor's dusk wings calldown with merc calldown from the topbar. Essentially you would get access to all the mercs available in the WoL campaign (excluding spartan company goliaths because Raynor doesn't have goliaths in his base kit), all on a shared cooldown. This means you'd need to choose carefully which merc you need for each situation. After 60 seconds, air units fly away and ground units are picked up by medivacs. The mercs come with all their upgrades and include:
War pigs (marines) - unlocked after building barracks. Calls down 12 war pigs. Useful as an early game boost eg. to clear expansions. Covers the calldown area in a hail of bullets upon usage which targets both ground and air units.
Devil dogs (firebats) - unlocked after building engineering bay. Calls down 6 devil dogs. Useful on infested maps. Covers the calldown area in fire which causes damage over time to enemy ground units.
Hammer securities (marauders) - unlocked after building engineering bay. Calls down 6 merc marauders. Useful for enemy comps with lots of armoured ground units. Covers the calldown area in concussive shells which damage and slow ground units.
Siege breakers (siege tanks) - unlocked after building factory. Calls down 4 siege breakers that land in siege mode and immediately shell nearby enemies. Great when you need siege units in a pinch.
Hel's angels (vikings) - unlocked after building starport. Calls down 6 Hel's angels. Useful against enemy air comps. Covers the calldown area in a swarm of anti air missiles.
Dusk wings (banshees) - same as the current calldown. Unlocked after building starport. Useful against ground armies without detection.
Jackson's revenge (battlecruisers) - unlocked after building fusion core. Calls down 2 merc BCs which have the yamato cannon and tactical jump abilities. They spawn with an active defensive matrix which lasts 60 seconds. Great for tanking damage and sniping dangerous units with yamato.
These calldowns will give Raynor a lot more flexibility and won't fall off in usefulness as badly as the current banshee calldown does in the late game. The exact number of units spawned can be tweaked for balancing purposes.
These new calldowns also provide an alternative prestige idea for P2, which currently doesn't provide much of a change to P0.
New P2: the merc calldowns are permanent (advantage), but cost resources (disadvantage), similar thematically to Alarak P3. The resource cost can be tweaked for balance purposes, but I thought it could be cool if they mainly cost minerals, essentially testing how good you are at using mules to maximise your mineral income. High skill players may be able to max out their supply with a pure merc army! If this is felt to be too strong you could also remove Hyperion for another disadvantage.
With this new P2 replacing the old one, the afterburners upgrade could be changed to boost both movement speed and attack speed for ALL prestiges (at the cost of 10 HP), increasing its general utility and keeping players who enjoy the current P2 happy.
Other units and structures
Diamondback and its upgrades tri-lithium power cell (+1 range) and shaped hull (+50 HP) - as many others have said before, this would definitely fit Swann's kit best.
Predator - this is a meme unit from the campaign and is too weak in this state to be useful in coop. However I think a hero version of the predator could be given to Cannonball as his pet dog Cerberus who follows him around and automatically attacks nearby enemies with a buffed version of the predator's AoE attack. This would give Cannonball a much needed buff, and would fit thematically - he is a Herc after all, and in mythology Hercules captured Cerberus. The Redline power cells gear would also affect Cerberus' attack speed. I'd also change Cannonball's ultimate gear to 'you killed my dog!' - every time Cerberus is killed, Cannonball is enraged and gets a 4x multiplier to his attack damage for 30s. Cerberus then respawns automatically after 60s. The idea is, Cerberus jumps into the fray at the start of an engagement and takes out the smaller units with his AoE attack, then when he dies Cannonball uses the rage-induced damage buff to take out the beefier enemies like hybrids.
Raven with seeker missile, auto turret and point defence drone abilities - the seeker missile is already usable by Nova's raven, and PDDs are already part of the Hyperion's abilities, so in order to differentiate the standard raven better from these, I'd rather give it the anti-armour missile and interference matrix abilities from the 1v1 raven instead (auto turrets can stay). This version of the raven would suit Raynor's kit best - he needs mobile detection, anti-armour missile would synergise very well with marines and BCs, auto turrets can stall, and interference matrix would give him another way of countering dangerous AoE spells like storm, parasitic bomb etc.
Spectre and its upgrades psionic lash and Nyx-class cloaking module (indefinite cloaking) - technically spectres do exist in coop in the form of Nux, but standard spectres don't really fit well with any of the current commanders, and in my opinion would be best kept in reserve in case we ever get a Tosh commander.
Planetary fortress - this would suit H&H best, as they currently lack anti-ground static defence. I would also suggest changing space station reallocation to 'planetary reallocation', dropping a planetary fortress on the targeted spot, doing similar damage to units it lands on that the space station does. This then attacks nearby enemies, and can be used as a forward base if it survives, eg. for luring enemies to mag mines or dropping one at enemy spawn points to support spawn camping. If destroyed it detonates a nuke, or you can manually detonate it if you wanted. I'd also give H&H the ability to upgrade their command centres to planetary fortresses.
Hive mind emulator - this is a great defensive structure from the campaign but it is hard to see which of the current commanders it would fit in with. I think this would be best saved for Tosh, but would need to be changed to affect non-zerg units as well.
Psi disruptor - as above, this would be great on defensive maps (though you'd need to extend its effect to non-zerg units too), but doesn't quite fit with any of the current commanders. Again I think this could work well for Tosh, giving him a very unique defensive setup.
Upgrades
Ultra capacitors (weapon upgrades also increase attack speed by 5%) - Raynor has its defensive counterpart (vanadium plating), so I think it would probably make sense for him to get ultra capacitors as well, as he is the commander most in need of buffs.
Micro filtering (25% more gas from refineries) - Swann's gas drones are effectively the same as this upgrade, so this is already covered.
Command center reactor (double SVC production) - I think this would fit Swann the most as he tends to build more SCVs than any other commander, and this would help him ramp up his economy more quickly in the early game. It should probably be an upgrade at the command center (similar to orbital command) so it isn't too overpowered.
Cellular reactor (+100 starting and max energy for spellcasters) - of the terran commanders, I think either Raynor (medics and banshees) or Swann (wraiths and science vessels) would be the best fit for this upgrade.
Dual fusion welders (doubles SCV repair speed) - Swann would be the natural fit for this upgrade, but to be honest I don't think he really needs it.
Advanced medic facilities (medics don't require tech lab) - I guess you could give this to Raynor, but I don't think it will be that impactful to be honest.
U238 rounds (reapers gain +1 range and +3 damage vs light targets) - this could help make H&H's reapers stronger, though of all their units I think their reapers are least in need of an offensive buff.
G4 clusterbomb (reaper ability) - this is a pretty mediocre ability, and H&H's reapers already have an on-death grenade ability which is better than this, so I don't think this really has a place in coop.
Twin linked flamethrower (doubles width of hellion flame attack) - this could help improve both Swann's and Nova's hellions, albeit only a little.
Rapid deployment tube (medivacs deploy troops almost instantly) - only Mengsk could use this, but medivac drops are not really that useful in coop.
Tomahawk power cells (+100 starting energy for wraiths) - either Swann or H&H could benefit from this, though it does overlap a bit with cellular reactor above.
Cross spectrum dampeners (banshees can cloak for twice as long) - Nova has a better version of this where her banshees can cloak indefinitely without needing energy, so this is already more than covered.
Missile pods (anti-air AoE ability for BC) - I think H&H's BCs would benefit most from this ability, as H&H can sometimes struggle with large numbers of air units. This would complement their vikings' WILD missiles ability which provides good anti-air single target damage.
Ocular implants (+3 sight range and +2 attack range for ghosts) - I guess you could give this to either Nova's or Mengsk's ghosts, though I'm not sure either of them really need a buff.
What do you all think? Is there anything I've missed from the campaign? I might do similar lists for the HotS and LotV campaigns too.