I'd love to run a mixed unit mercenary campaign starting in the early 3010's and maybe running all the way up to the Clan invasion, some multi-generational type campaign.
One way I started a campaign like that was basically 'Mechwarriors' that were piloting IndustrialMechs vs various pirate raids, set in the FedSuns' Outback. They are in decently heavy Mechs so are loaded down with armor (enough to survive a few hits), and the pirates are in a few badly patched Mechs with mostly trucks to pick up the loot.
A few raids like that allow the PCs to upgrade their Mechs to something combat-worthy, while RPG schmoozing allows the PCs to get better access to local resources. The regional lord can visit and allow the PCs to get access to their technicians and some advanced weaponry, aka the GM helping the players out ahead of a major fight.
Eventually, the PCs have BattleMechs and some decent skills, and have good relationships with nearby House units that help protect vs pirates. Then the 4th SW occurs, and House units near them are pulled out to help cover areas, meaning the PCS now have to cover more territory from pirate raids. At the same time, there are Star League weapons being introduced, meaning the older weapons are steadily being removed and put into warehouses. A few supply officers are corrupt, tag several of the weapons as scrap, and sell them to cut-outs who get the weapons delivered to the pirates, while the supply officers get a nicer retirement package. The players now have to deal with pirates that are better equipped, in addition to their forces being thinner spread.
Eventually the merc unit manages to get the location settle, unfortunately it is now 3049 and more House units get pulled off the line to deal with a problem near the Rasalhague Republic. More pirates start showing up again.