Plenty of great ideas here from previous posters.
I concur, armies are a very lackluster overlooked and underloved aspect in a game 4x grand strategy game such as Stellaris.
1st off. Customizable armies. Tech unlocks, either specific or tethered to existing tech research should open up attachments for troop types. Troops can unlock with a "generic" set of traits, or also be completely open to customization by the player as though they were a subset species (kinda like how you can tailor crafte gene's an robotic traits). Mind you these would be minor tweaks, but could flavor flair to suit a players specific species concept. At the very least troop types should have a build interface similar to your ship sets. Renaming, an fitting them in a standard preset based on their role function, so when you build them you don't need to mess with any attachments. Depending on the species and tech unlocks for advanced ground troops should open up the range, number and tech style of attachments allowable per unit.
2nd more troop styles. There's a few mods that add in various new styles of ground troops, from large mechs, to aerospace and the likes, they're simply just a name with set attack strengths an health. No actual mechanics beyond that, or any tech system to enhance or specialize them.
I would love to see additional unit types, such as mobile ground carriers, armored ground transports, armored battle tanks, aerospace, VTOL mechanized infantry, Light and heavy exo warframes, Orbital Drop Assault Troops, Special Force Commandos. As well suitably subsequent variants that would come from species that are of a non-human variety, thinking in the lines of aquatic life, or fungoid, biologically driven gene consuming tyranid/alien type species.
While I would say it would be a bit much to ask for, an quite possibly too much to micromanage. I would like to see an extended ground combat that while it would play out on it's own, but, could have an interactive interface for the player to take a more active hand in determining or speeding up the outcome of the actual fight.