Personally I would prefer to see initial releases concentrate on a few core troop types from either side allowing skirmish games fairly quickly, so next would like to see Foreign Legion, Contre Guerillas, Juaristas, regular Mexican infantry, Rurales perhaps later adding some more specialised troops, characters etc.
At some point I would love to see some special pieces, such as baggage train elements that can be used to create scenarios. Attatched picture of Cantinieres that could be useful if you go down this route
