I would like to have had some skills unlock as the unit gains veterancy.
So at level 1, for sake of argument let's say it starts with something like: single direct fire shot with some combination of: range penalty, height or terrain or walls effects, object occlusion, and/or "move and shoot" penalties. Simulating a novice archer, taking direct aim the way he used to do at womp rats back on the farm.
Level 2 unlocks indirect fire mode, avoiding light occluding obstacles including low unroofed walls and intervening troops, but at cost of a steeper "Scatter" penalty to damage proportional to range
Level 3, passive, reduces Move and Shoot penalty
Level 4 unlocks opportunity fire, holding fire until the enemy makes an action, but then it ignores some fraction of local terrain cover penalties (designed to counter ENEMIES IN COVER)
Level 5, passive, reduces Scatter penalty
Level 6, unlocks suppressive fire, reduces the initiative of enemy unit, perhaps at the cost of some damage (designed to counter single HEAVY HITTERS)
Level 7, passive, reduces vulnerability to ranged damage if the unit didn't fire the previous turn (incl. turn 1)
Level 8, unlocks sniper shot, ignores friendly unit occlusion and hits for +damage any enemy engaged in a melee
Alternatively for AI/user simplicity you could have all these active skills available from the get go but they are reduced in effectiveness until you level up and gain an appropriate trait.
Various unit traits could selective boost one aspect of the archer's role over another; starting with 1 level could be important to get that first critical "professional" ability (in this example, indirect fire).
Various items could counter certain enemies' skills and abilities, or adapt to different terrains. Caltrops to mitigate cav charges, targe shields for cover at range, firebox to enable damage to structures. The impact of enchanted bows, rather than flat +dam it could channel elemental arcane arrow damage proportional to number of the matching shards controlled by the empire.
... etc. Maybe this goes too far towards wargamey grognard complexity, or maybe my specific ideas aren't perfect. The exact details aren't the point, the point is (a) any number of more tactical 'hooks' could add meaningful decisions to the unit design and empire building strategic layers which IMO are lacking ( b ) different types of units could shine on different battlefields, promoting combined arms in an organic way (c) for fun factor within the tactical minigame itself, battles should sometimes last long enough that multi-turn spells or tactical maneuvers are desirable. Or simply, that it matters which spell you cast. Locations with heavy cover like forests or walled towers are one approach to making that happen, but then you want (at least veteran) units to have the tools to counter these situations.
I sort of like where is coming from the idea of the big axe cleave and the spear lancing thing ... the problem is that in the absence of too many other traits, abilities, cover, counterattack by defenders etc. coming into play they, the net result is to just front-load everything into turn 1 of the engagement. IMO those abilities are too powerful and should be reserved for some kind of veterans or elites (maybe horse-spearmen and horse-axemen only) while common infantry get different abilities that aren't so front-loaded. (attack at range 2 or immunity to counterstrike for spears, swarming/flanking bonus for axemen)