So, thinking about Wizard Tactics, I've come up with a few fundamental game play concepts that will influence everything else.
1. Movement and actions
So there are basically three ways to handle this. The first, as seen in things like the original Xcom and Divinity 2, is Action points. Basically, every action, power, and movement would have a cost and you could do as much or as many as you wanted so long as you could pay it. The second, as seen in the recent Xcom and a lot of other more contemporary tactics games, is the idea of having 1 move and then an action or two moves with no action, or an action alone. The last is the Fire emblem or advance wars style where you move once, and then attack or just attack.
I would go for action points.
2. Classes
So in the original xcom, if memory serves, you don't have any kind of classes or restrictions. Anyone could use any weapon or item and no one had any special abilities. Divinity 2 is...kind of like that where any character can use any spell or weapon or item if they have the right stats or skills or attributes or whatever. There's a lot of freedom in that, but everyone also feels very interchangeable if they're not specialized or have some sort of defining trait. The alternative is to have set classes with set abilities, items, weapons, etc that they and only they can use. It means you're more restricted on how you can act, but it also means that each unit has one thing they're good at and can contribute in a meaningful way.
I'd go for Classes.
3. Micromanaging
Original Xcom and Divinity 2 have a LOT of micromanaging. Managing each armor piece, each magazine, each gun, each grenade, each item, etc. Meanwhile the recent xcom, and a lot of tactical games, don't bother with that sort of thing. Guns just have infinite ammo, though they need to be reloaded every so often, and items are generally very few and are replaced automatically upon mission end.
I prefer minimal micromanaging.
4. Permadeath
All the Xcoms and Fire emblem have permadeath, at least in terms of units being able to be used. Divinity 2 has death but it can be undone by resurrection spells and scrolls, and some tactics games just have characters "Faint" and get brought back upon the level ending.
I prefer permadeath