I'm reminded of Zone of the Enders: Fist of Mars actually. A little-known GBA tactical RPG (no grinding, similar to most Fire Emblems) that was an interquel to the fast-paced mecha fighting ZoE 1 and 2.
Looks cool. Thanks for hinting at. Also, GBA had actual XCOM-like game from Gollop, called Rebelstar. It was heavily stripped down to make accessible for younger audience and play nicely with GBA hardware, but still has destructible terrain.
If you go with 2 or something similar, I vote for an optional evil campaign. The gaming world is starved for good evil campaigns.
I decided to mix it all. The king has died and there is now a succession war, between different sides, with an impostor capturing the throne and taxing population. That scenario allows for the most freedom and any number of sides. I've also abandoned the idea of linking player character to be a wizard, so now player can promote any unit into a leader and have any number of leaders.
The development took some really wild turns. Initially the game was just a Chess clone with some modifications, like Sokoban block moving, but then I found that small maps are very limited, so I abandoned the Chess like mechanics, adding HP, but retaining the no RNG paradigm. Then I found that my engine is a bit inflexible, it would be had to impossible to implement proper cutscenes and more complex animations, so I turned the engine into realtime RTS one, with turn-based limitations just added on top. For a short time I played with idea of making a dwarf fortress/dungeon keeper like game, adding room building and cave excavation (the imp character was originally used for that), even some initial resource gathering, but I found it boring and overused, so I kept it turn-based, but retained the terraforming part, because I believe it is cool to chop down trees and make a raft to cross the river, if player has no flight or water walking spell, or any magician at all, or to dig a tunnel under a prison to free prisoners. Anyway, now I have clear idea how it all should combine into a solid game.