I believe Penal Legion Squads to be a good fluffy choice. There is no need to model them after convicts and criminals. You can have them as your fearless, Bushido trained Bansai-chargers fanatically loyal Infantrymen.
At least, something like that is what I actually do...:secret:
Priests and Commissars could be a fine idea and remember, many Japanese soldiers thought of the Sniper Rifle similar to a 'long-range Katana strike', they utilised Kendo and Iaido techniques and philosophy...the patience, the discipline, the one-shot...one kill! Sharp and calculating...the Japanese way.
Primarily famed for the imperial Navy, you could have a Valkyrie or a Vendetta and paint them with your Homeworld's symbols or Rising Suns and as far as i know, Japanese Armour Divisions were somewhat limited in both quality and quantity, though I think they did have some innovations like Diesel Engines...(am I wrong here? I apologise) Thus you should restrict and limit your Leman Russ options.
I have been thinking of the HARAKONI Warhawks as somewhat Anime...maybe the sound of the name, the Carapace-Armoured Infantry, deep striking and daredevil...and even had this instict that GW would have 'Samurai-sed' these Carapace Armours and fix some Katana shaped CCW and release them as the Anime Guardsmen :male-fighter2: