While it may be tempting to try and either modify X-wing or Imperial Assault to make a ground combat game, I think we should rather look towards Armada. Though we shall discuss more of the similarities as we go on.
Here are my ideas:
Initiative, Missions, and Planets
Just like Armada, I believe an objective system would work very well with the game. Yet unlike Armada where combat takes place in space, in ground combat we have a larger verity of obstacles and environments to overcome. This brings us to the topic of planet cards, therefore players will be required to select 3 objective cards as well as 3 planet cards. These planet cards would list the number and types of terrain to add to the match as well as any environmental effects that would take place on specific turns.
The core set could provide 3 planet archetypes say Forrest, Desert, and Plains with 3 to 4 terrain pieces for each provided in the core set. Further expansions could add even more planet cards as well as terrain allowing for battles on snow and city planets as well as giving more options for the existing ones. The various terrain tokens could potentially give bonuses for attacking, defending, increasing or decreasing range. Ultimately the point being that it would allow varied battles requiring players to take advantage of both the special rules provided from the objective and using the terrain.
As for determining who selects which we could fairly simply follow the rules in Armada with the first player selecting on of the three Objective cards from the second player
Infantry and Vehicles
The images below show my basic idea of what the cards may look like for both infantry and vehicles. From these we can see an obvious Armada influence in regards to the layout and a fair number of other key aspects. For infantry they could move similarly to how squadrons do in Armada though vehicles would better be suited either to a maneuver template similar to Epic ships or some new maneuver tool. Of course once air support was added to the game they could simply reuse the X-wing maneuver templates, though more than likely having the craft have a top speed of 3 or so.
As for the attack dice, my idea was breaking it down into light and heavy attack dice, represented here with the Armada red and black dice, though it could certainly be played with more to come up with new ideas. The action bar in my mind seemed like a good idea, though may clutter the table with tokens, with some of the actions being Charge, letting you move again, Aim, letting you reroll a die or be used as a pseudo focus, and Take Cover, reducing the damage you take by one.
In regards to combat though I kept debating over whether it should be done like X-wing where everything moves then attacks or alternating activations. For the first it would probably be something like infantry moves, vehicles move, and then air support moves and then the attack order is the reverse. With air support ships still being able to perform actions when they would bump with infantry or vehicles with the only negative being that they can’t target anything their base was in contact with.
Infantry would lack evasion dice, to better emphasis using the terrain tokens to their advantage and that they would be able to maneuver themselves around the firing arcs of vehicles. Whereas the vehicles would have both abilities and defense tokens to better prolong their stay on the board.
Lastly upgrades would for vehicles and air support would be pretty similar to X-wing maybe with some cross over in the use of cannons or other items. Whilst Infantry would be limited to Equipment upgrades which would be items like Rocket Launchers to add heavy attack dice to their Vehicle attack, or Environmental suits to mitigate negative effects the planet card may throw at the player like sandstorms, rain, and blizzards.