In my spare time I often find myself designing games in my head. One that I keep coming back to is, like your idea, a fantasy X-Com-like party management game.
Our ideas will quite likely diverge quite a bit as some of the things I've cooked up lean towards the absurd. One semi-rejected idea is a class that disbelieves mythical races and creatures out of existence.by virtue of concentrated disbelief and (likely unjustified) logic.
It's a bit of a generic answer, but a big part of game design is of course to find a way to hook the player in to keep playing. If you can come up with a fun game to play, then most definitely.
However if it's it's overly dark, grim and gritty with taupe as the primary colour scheme - then I might pass.
But since im also planing to make your units "inmortal" (they can be resurrected on cities) losing a combat isnt a matter of life and death, its a grindy mechanic im inserting to get the player to engage and level up their troops, losing will have its penalties of course, you will have to wait until the units you took are healed back and forced to take others meanwhile(possibly lower level or class), loss of materials....
One idea would be to provide some handicaps to the returning unit that will gradually disappear over time or after a certain number of battles. Imagine a fighter with one arm in a sling who is still able to fight with the free hand but cannot wield large weapons or a shield. Or has a sore leg that causes movement to drop by one hex. Injury dependant on what caused the character to fall in the first place.
A more severe penalty would be to lose an entire character level after being revived.
So, i am conflicted about adding a grindy gameplay mechanic in there, you need the grind to upgrade your units and get a nice rooster troop (heavies, lights, fliers, siege...) ready for each occasion, my way to fix this is unlocking the class promotions by research and trying to give the progression a nice pace.
It should be fine as long as what you come up is balanced and works consistently within the scope of your game.
It's not for everyone but some gamers like myself quite enjoy games that involve a bit of grinding. One of my favourite go-to strategy RPG series being the Disgaea series which has grinding as one of its core game mechanics, at least for its post-game content.
However grinding has to be meaningful and reward the player for their efforts. If the grinding is reduced to just churning the numbers with minimal or inconsequential effect, then it's not enjoyable at all.
How much customisation are you planning to allow for each character as far as their abilities and skills go as the characters grow in experience? If there is going to be grinding, you could use it as a means of developing a variety of different skills on top of the profession specific abilities. Such as mastering specific weapons, first aid, foraging, riding, equipment maintenance, etc.
NKF, narrow minded fuddy duddy who refuses to let go of the past and will not accept anything newer than 1979.