I am rather fond of the Barracudas-segment, it was a good atmosphere: The GTVA trying to adapt to Nebular warfare but Shivans are still orders of magnitude better at it - it kinda hammers home of the point that it is not a question of tech anymore. ...
A Game of TAG only issue is that you are forced into the most sub-optimal fighter for that particular mission (take a fighter with better secondary placement so you can aim at something and it would be swell)
High Noon does its job rather well for the standards of way back then - putting the action in a cutscene (considering the mission is actually just a big cutscene by today's standard) would defeat the sense of scale you get from the Sathanas and the Collossus. I makes you feel tiny in comparison to the forces arrayed by either side and it helps scale the action in the later half of the campaign.
Their Finest Hour suffers from a problem that the Collossus doesn't scale well to engagements, other than providing a quick killiing blow, with targets other than a de-fanged Sathanas. It's just too overpowering, so

made it completely useless. If you found a way for the Collossus not to dominate the mission and keep it consitent with the rest of the campaign, that would be change worth seeing.
As for a mission I would really change, it's
Slaying Ravana. The original is supposed to be the end of protracted battle, with the Raven's doing the final bombing dive because the GTVA's forces are spend. Sadly nothing really feels that way about the mission; that part a fault of these dive-bomb bomber missions really not being that great (tons of understandement here).
Personally the initial segment of the battle would be far more interesting, with the Ravana engaging a GTVA task force with a ship of it's own weight at the centre. Maybe not fully base the mission on an attack run on the Ravana but on how the GTVA task force has to scamper away from the Shivans while the Delacroix and the Ravana are locked in deadly close quaters combat at the end of which both ships deliver the each other the killing blow.
(it would also work for a tactical standpoint since as soon as you have closed out of the firing arcs of the LReds the Ravana get's kinda toothless with its 2 SReds on top)