I'm wondering why are You spoiling us all the storyline U have...
No matter what the storyline is. If You REALLY have to make 50+ long campaign, divide it into acts and release in 20-30 mission long chapters. I could release my current project as a 100-mission long campaign, but it would take years. Instead of, after a year of hard work we have almost ready Act I with 25 missions. Isn't it better this way?
The other thing is importance of individual missions for your plot. Derelict for example has a lot of missions fulfilled with random skirmishes with pirates and some other boring stuff, which are not needed for the main storyline. Aftermath and Shadow Genesis have similar problem. It's good to hold the twist of action for some time, but 5 missions with random patrols and raids on illegal pirate cargo depot before would kill all the fun. Don't make your campaign longer than it needs to be.
I thought of making acts already,and i will make them.I'll also try shorting the campaign a bit,and i will also add some trainings.And change first mission completely to make it more ,,playable'' and it will be kinda like fleet battle,to enhance first impressions.Acts i am thinking of:
Act 1:Attacking Shivans in and beyond Fomalhaut.10 missions.
Act 2:Shivan Counterattack in Capella,stopping assault on the Nebula after Fomalhaut,retreat to Delta Serpentis,Should force in Vega fail.6 missions.
Act 3:Shivan attack on the Ross 128.Vega force retreats to Delta Serpentis as well,losing Icanus.Unknown number of missions.
Act 4:Final attack on Sol,eliminating last Celebrai ships.Gigas makes it to the planet with moderate damage,gate opening Shivans escaping to their home.
That's current idea,during creation it may actually be changed a bit.