Norman cavalry to the same high standard as your Mini-series infantry would be perfect. Also firing lines for the ACW to complement your ACW command sets and provide them with "cannon fodder".
Advancing and firing Streltsi would fill the gaps left by Zvezda's set.
Please continue making them. I love collecting them. Where there any strelets personalities? Perhaps you could make them as a personal touch. Or you could add personalities from Russian history (Catharina the Great, Ivan the terrible, Peter the Great,...) Or just personalities that are linked to your sets(current and past)like William the conqueror, more Napoleonics, Crimean commanders, famous knights, Romans,...
Or just extra poses to previous sets.