So let me propose two changes that shouldn't have a noticeable effect on balance nor the skill-cap, but makes it easier/simpler for the average terran player:
1. Reduce the control group priority of the Raven. So if you have selected bio units or siege tanks along with the raven, the tanks/bio takes priority. This means you can still stim or siege up when needed.
I can't be the only terran player who has lost battles because I couldn't stim "up" when I had to and instead had to first alt-tab. Terran players can instead have the Raven in a secondary control group that they only use when they have to cast its abilities.
2. Give Ghost a version of stim that costs energy and adjust its core stats based on that (e.g. lower base MS and DPS).
This change will make controlling Ghosts so much simpler since you do not need a secondary control group. You can mix in ghosts with your bio army and have all of the units in the same control group while both stimming and casting Ghost abilities.
Some may argue that these changes reduce the skillcap and yada-yada. I don't really think that's the case. I think its more of a skill-floor thing, but regardless those changes seems to fit Blizzards new design philsophy so I would like to see terran receiving some as well.
At last, I should point out I have actually tested both of these solutions (in the editor), so I am not just speculating that they will make the control feel smoother.