Leader vs Dual Clans

Oh, just read the Bushiroad post. Looks like they want to go with Lord.

And it's meishu. It does mean leader, but they're probably trying to tie it in with the anime where the clans are trying to create a new alliance kind of thing. The other stuff you mentioned like expert, skilled master are also meishu, but the kanji is different so they would not apply to this instance of 盟主.

I can't see the tiny tiny kana above the Kanji, so I was retroactively trying to make it fit. If they're going with Lord, it actually seems more logical to use Myoushu. Especially since Bushiroad have a habit of using outdated Japanese words. Though, I suppose they don't actually use them in effect text only in flavor portions of the card.

But yeah, it's most likely the alliance part of it and the fact that there's really no exact duplicate word of that concept in English. Lord works what with the land owning theme of the Clans.