First of all, more melee-based. Make the units closer to melee, to help bridge the gap. Give BCs cooldown-based abilities and let them attack on the move, give marauders their +25 hp without kinetic foam, condense vehicle and ship armor into 1 upgrade, etc.
Obviously, missions would need to unlock different units:
Outlaws, instead of giving medics, gives orbital command (purely for mules for now. Orbital command in armory upgrades is replaced with the addition of scanner sweep).
Evacuation gives hellions. Have the convoy be multiple trucks long (probably with more health each) or move faster. Hellion mobility would help cover the trucks, or keep up with them.
Outbreak gives hellbats. Pretty self-explanatory. Good defense during the night, and you’d have hellion upgrades like Brue frame to burn down stuff in the day.
Welcome to the jungle gives widow mines. Another weird one, but the Protoss always attack in a wave, so widow mines could be very good here. Protoss would still have no observers, but you don’t get drilling claws yet so it kinda balances out.
Breakout can stay the same. Spectres make the most sense, even if they aren’t melee.
Great train robbery gives Cyclones, for obvious reasons.
Cutthroat gives medivacs. Cost efficiency is the name of the game, and medivacs are good for sustain and mobility. (Makes up for the lost spider mine defense and vulture mobility).
Engine of destruction, if the player hasn’t already done Haven, gives vikings. If they have vikings, it gives them smart servos. (Also works vide versa, with Haven giving smart servos). Alternatively, gives liberators, with the main threat to the Odin being tons of banshees instead of battlecruisers.
Lab can mostly stay as is. But some things should get buffed. Drop pods should be changed to affect all production, raven should change to present day raven, Hercules should be able to tank drop, predator should get more health, armor, and (new) life leech.
Mercs can also stay as is. Maybe make merc compound 3x3, for sake of consistency with EVERY BUILDING EVER.
Secondly, some mission redesign:
Evacuation, smash and grab, either give them feats of strength for genocide, like with welcome to the jungle and devil’s playground, or make it more clear that you shouldn’t be attacking the zerg base. It seems obvious already, but I’ve seen plenty of newer players suicide into the zerg base for some reason.
A sinister turn, make the preserver cells more spread out. Maybe have 1 in the expansion, 1 right before the light bridge, and 1 at the end of the map where it is now. With each prison killed, the hybrid takes longer to respawn. “We must save the preservers before it’s too late!” would make more sense gameplay-wise.
In utter darkness, give the Protoss present day things like shield batteries and phoenixes that attack on the move and tempests. At the very least, make carrier interceptors cheaper or even free.
Also give protoss chrono boost.
All in nydus variant, send in a few omegalisks or brutalisks or something leviathan-equivalent. Make the choice between air variant and nydus variant a little more difficult.
Pretty obvious none of these things are going to actually happen, but it would be pretty interesting.