A few days ago we assembled to commanders at my place and fought a Battletech scenario. BT is old school to me, whatever rules you use. My boxed set if the 2nd ed.
Along the border of the Steiner and Kurita empires, worlds have started to rebel, not respecting the deal by Theodore Kurita and ComStar. (this is before the clans. I did say old school, didn't I?). One such planet is Zyclone 3, where the samurai mechwarriors get their orders from the local warlord, that coming down from the mountains are two lances of Steiner mechs. They are spearheading a push toward the industrial centres in the lowlands. "We are redeploying to meet this thread, but you have to Hold the Line until reinforcement arrive and we can defend these vital resources!" Let the battle commence.
I played the Kurita guys, and had one lance of veterans in medium mechs. The Steiners had one light lance and one heavy lance. They were regulars, and the light lance mostly so. Naturally, the first thing that happens in that the heavy mechs stumble and fall while crossing a river! Some giggles in the light lance when that happened. Then I got my LRM and autocannons up on two hills with good visibility on a majority of the battlefield. Missiles started to rain as soon as the Steiners came into view. Two batteries of LRM 15, firing each round for four rounds. Just picture it!
In the end I managed to blow up two enemies, and shoot off the leg one another. It looked like a victory and the other player yielded. This was so fun that I at once started to think about doing a campaign. Naturally that makes you start to think about stories, free form developments and sandboxes. Again.
There are different rules for Battletech campaigns, and some of them have tried to handle the fact that you might want to play in a story line, but also to be the master of your own fate. I have read some of those and been thinking that maybe they can be used got a RPG campaign?
In a BT campaign you use something called tracks, which specify not which troops show up, but their relative size and strength. Also, the terrain is stated in a general case and the outcome can lead you to another track depending on what happened.
If you want to try to game a more controlled story, I don't see why you couldn't use something similar. In Traveller adventures, at least those from DGP, utilized something called nuggets. A nugget was a few resources for an encounter, and it had a dependency tree, i.e. it was connected to nuggets that had to happen before that one, and nuggets you could go into depending on the outcome. I think I'm going to go back and reread some of those old Traveller adventures and see how it worked.