this is not meant as a "please do this", but merely as a general spitballing type of post and maybe a reminder to not forget a raw idea
How about a classic fairytale-type kind of story:
It starts with a basic problem: you have an adversary that outnumbers and outgunns you. But you need to fight him for whatever reason (freeing a hostage/town/continent/planet, or its an ursurpator that killed most of your family and you need to gain your rightful property back or something).
So starting point would be you escaping (infantry/power armour or a light mech).
Then you are on the flight with the need to gather resources and allies (=pilots), which would be about 50% of the campaign.
You could involve a scavenger hunt type of story to get some special equipment, about another 20-30% of the campaign.
The last part would be actually taking back whats yours (you could limit the drop weight at the first missions of this stage due to some reason ... speedy shuttle needed, 2nd jumpship technical trouble (or dropship if a 2nd Jumpship is too much out of scale) and delayed.
With the final battle against ... well you get the point.
Nothing of this is new, some of it i read in other posts/forums and i don`t want to take credit for the idea.
Maybe many things aren`t actually possible with the engine (e.g. strength of opposition partly dependant on your forces (drop weight?) or a tree-like campaign part, where missions available depend on your achievements in prior missions).
In my head it would be a campaign of 30+ missions, variable in size and, depending on your descisions you would not even see all of them in one playthrough.
I know this is a lot of work and propably too much work for one person ...
so at least for now its just an idea i wanted to write down