So what would the best kind of purchase be for the new player to get after the box set? In many ways I am imagining something similar to the expansion packs that were made for HeroQuest: new miniatures, items, weapons, map counters, and battles. BattleTech has already done similar things, but not all at once. So maybe combining the idea of the lance boxes, map packs, and scenario books all into one product would be a good idea.
So you take a merc unit or other group. The simple go-to is Gray Death Legion. You tear apart and remake the old scenario book. You include new 'mechs to cover what was not included in the BattleTech core box set: both miniatures and stand-ups and maybe you include a code for some percentage off an online purchase at IWM. You have new maps to introduce new terrain that can either be general or appropriate for a specific scenario as well as terrain tiles or building tiles.
One of the things that struck me about BattleTech is that you had heroes or units that you could follow through years of products. Wolf's Dragoons and The Gray Death Legion in specific had multiple scenario books, novels, and mentions across products. Because of the lack of overall fiction for the past decade, players lost that. The closest we've gotten were Devlin Stone and Stacey Church. Bringing a bit of that pulp back is probably a good idea to attach players to the universe.