For starts, I really want to see a change to Siege Tanks. Mainly a gas reduction to 100gas, and a supply reduction from 3 to 2. The build time change in patch 8 is okay, it was another change I was hoping for and while it isn't a large change (5seconds) it's a nice improvement. The gas and supply are, as I see it, the two big factors in making tanks, especially early game.
Another mech change, a slight one at that, is to improve the attack animation delay on Hellions. As it is now, there's a pause between when they attack and move. The delay between attacks is fine, just being able to smoothly go from attacking to moving which should improve their ability to be microed. It's possible but it would feel much smoother without that slight delay.
Also, it might be nice to see a base damage increase to 10 from 8, and a reduction vs. light of the same amount to balance it so the overall damage vs. Light is the same, but base vs. other armor types is increased slightly. If that isn't done I'm fine with it, as I personally love Hellions. They are my favorite Terran unit and I use them all the time, but it would be nice to see more damage vs. non-Light.
Next up are a few minor changes. Having the Engineering Bay able to lift-off would be really, really nice. It's a small change, but having a floating building that isn't a production building to scout with would be nice. It even looks like it can lift off, but alas, it cannot.
Salvage on bunkers is a nice feature, and it would be great to see it on other Terran buildings as well. I've thought about salvageable turrets or add-ons, but I'm not sure if that's capable of being balanced, so better off left out.
Salvaging depots would be interesting, as it might give supply drop a use in late game. Salvage some depots -> supply drop on others to keep the cap. Since there's no depleted mining anymore for gas, it would be nice to salvage refineries. It's not a huge change, just something to add and have to do once you mine out a geyser.
For add-ons, every time I try to make my building placement neat and perfect, I get annoyed at certain holes that can't be filled. It's a bit of a quirk that I have with building placement, but it would be interesting to have the option of the add-on being on the left or right of the building. I'm not sure if it would look right aesthetically on the building, but it would make placing things neatly a lot better.
Other minor changes is the load scvs option on PF's and the lack of loading scvs on Orbitals. I see the point of it on Planetary Fortresses, but it's just not all that useful, at least I haven't found it to be. It would be great to have the loading for orbitals, since it makes more sense to me to have lift off and load to take your workers to safety. Whereas the Planetary is already defending them, and since it doesn't have lift off, if it dies the scvs are screwed anyway.
Well those are a few minor changes that I'd like to see as a Terran player. The only changes I really, really want to see are the Siege Tank change (or some type of change) and lift-off on e-bays.