I remember reading somewhere that they had worked on an RPG but dropped it and released dom5 instead. I haven't kept up with any of their media accounts did they ever pick it back up? I feel like with just the resources they already have they could make such a good RPG/rogue-like. exploring random lands with monsters(COE4)
delving into dungeons for treasure(dom5 the random site)
Magical items & spells (COE4 & dom5)
Random world ending events to spice things up(COE4 & dom5)
Unique monsters for every terrain (dom5 has so many units)
Bosses (Mostly dom5)
I feel like it's all there already. Heck they would just need to slap an inn on all randomly generated major cities and allow players to trade adventurers or hire them out and you'd have a decent multiplayer trading economy.