A low fantasy XCOM/CK2-ish game in which you form a band of protectors who protect the people from Bad Things, be they supernatural (dragons, vampires, rogue wizards et cetera) or mundane (bandits). You can choose whether you'll support a single kingdom or be indiscriminate, or whether you'll support peasants or nobles. All of these things affect your standing with both the people of a given nation and the nobility.
As your forces grow larger, some of the kingdoms may fear your power and influence so you have to play the game of diplomacy with them, either doing them special favours or granting them concessions (allow one of their nobles to be privy to your organization's secrets, or be forced to build and maintain an outpost in an area that does not need the coverage, giving off your troops as bodyguards or mercenaries), thus becoming an actual player in the grand scheme of things. This diplomacy game replaces the airgame from XCOM.
Combat would be turn-based, take place on a grid and with proceduraly generated maps. Facing would be important in melee combat. You start off with 6 people per squad and work your way towards a maximum of 10. Combat prowess should rely more on the skills of your dudes and any buffs they might have than on fancy weaponry and armor..
Your soldiers can be either melee-based fighters, rangers or mages. Mages are basically what psi guys are in XCOM, they come in later in the game, take a lot of time and money to train up, but are capable of wrecking half of the map with their magic. Using mages should give you extra distrust from both the people and the nobles (since they fear magic) but the nobles should prove mighty interested in taking them as mercenaries or bodyguards.
The tech tree would be based on alchemy (to buff your soldiers with potions), travel speed (start off with horses, end up with flying carpets or something), more spells and infrastructure (outposts to extend your reach, workshops, research stations for magic, and so on and so forth), with weapons and armor being less important in the tree.
The entire world map is be pre-built, but the different factions you have should have different holdings at the start of each game and should have settings for different behaviours to increase replayability. Different events take place to which the nations of the world react differently.