And this is just an idea I'm gonna throw out there to see what people think:
High-end samurai heroes/summons: You said you want to have the emphasis be on samurai being rare, elite soldiers, and since a quick trip to wikipedia shows the Heian period as being when the samurai took over Japan and started getting big on bushido, why not work some bushido-ness in some way. I'm thinking, create one summonable thug or hero for each of the
seven virtues of bushido. This can be treated like summoning elemental royalty, where you cast one spell and get a randomly selected unique unit. This could help fill the void of some of the higher end oni summons that LA Jomon gets and add some extra flavor to the nation. EA Hinnom already does something like this with their spell
Release Lord of Civilization, though I'm sure we'd want these summons to be lower research and less world-shattering.