I really think BF should be extended to include Companion, but why stop there? Let's develop material for the Masters as well. Personally, I don't care anything about the Immortals type game, but that's just IMO.
What I like about these classic fantasy RPGs, is that the nature of the game changes as PCs level. By levels 4 -> 8, it's no longer just the dungeon. You're exploring the campaign setting and delving into the wilderness. But, after that, the PCs aren't just explorers. They're conquering the wilderness, changing it into civilized lands via their own baronies, fiefs, etc. They're beginning to alter the GM's campaign on a larger scale.
Having said that, much of the original rules in BECMI aren't really useful. The intended direction and scope described fires the imagination, but the actual implementation never seemed workable to me. The old Companion rules need to better define dominions. The mass combat rules are more solid, but could still use tweaking and consolidation with some of the later rules, such as for siege warfare and naval battles.
So, what would a Masters level ruleset describe? At this stage, the PCs are heroes of truly epic scale. They're actors at a global level, interacting with bizarre forces from strange dimensions and fighting wars against mighty imperial armies. PCs aren't worried about something as small as a village in their domains anymore. They're working to build kingdoms and empires of their own. They're staring down dragon kings and fighting evil demi-gods.
I remember reading somewhere long ago that dominions and mass combat are D&D's unfulfilled promise. It's past time to bring this epic scope into focus. And, yeah, they still go into dungeons.