One of the important visual identifiers of a Samurai army is the Mon, or heraldic badge that a lord would use to identify his troops and retainers. This would be displayed on the sashimonos, banners and even the helmets and breast plates of the ashigaru foot soldiers.
I could obviously copy a historic mon used by a famous daimyo ("great name" or warlord). But making up my own has greater appeal.
First I was thinking something based on cherry blossoms, referencing the infamous comparison between a samurai's life and the cherry blossom (both are beautiful and short). A Google image search began.
This graphic has several very cool, and very mon-like designs. But I worry the circles could be a total pain to do freehand over many dozens of banners.
This would also be possible. A simple five-petaled flower (leaving out the stamens of course). Actually I'm pretty sure I've seen something similar in my copy of the Osprey Samurai Heraldry.
Then I thought something to do with tea would be good for a confirmed tea-addict like myself. The Chinese/Japanese character for tea is this: