Wow, I thought I was the last to reply to this thread, but it just got pushed to the third page of my new replies list. Don't I feel silly.
To answer your question... well, that could involve a lot of explaining but the basic rules I'd considered so far were quite similar to Warhammer / Warhammer 40K. Yes, the game is currently gridless although having a grid isn't out of the question.
Warhammer
A basic run down of those rules would be that each player turn is divided into a number of phases. There is a movement phase, a shooting phase, then a close combat phase. Warhammer adds a magic phase, where 40K just has psychic powers work during other phases as appropriate.
Movement involves picking each unit and moving them in any direction (in general) up to the unit's move distance (always 6" in 40K for most infantry).
Shooting involves picking a target, and rolling a die for each shot fired, then looking the result up on a simple table of ballistic skill (BS) to see if it hit. If it did, then the weapon's strength (S) is compared against the target's toughness (T) in another table, and another die is rolled here to see if the target is wounded. If a wound is scored, the enemy must make an armor save by rolling under their armor value, which may be modified by the weapon. If the save is failed, a wound is removed. If a model loses all of its wounds, it's a casualty.
Close combat is a train wreck of exceptional circumstances and complex hit allocation rules, but basically follows as above.
If enough models are lost in a unit, it must roll under its leadership value or flee. If enough models remain, the unit may rally in subsequent turns.
What I considered:
There's just a single phase, and each unit must choose what to do: move, attack, use an ability, or possible other stuff (bunker down in their position, for example).
Due to a lack of creativity on my part, I was going to follow the same sort of idea as 40K, except that I'd be using d10s instead of d6s, have an open ended stat progression (in 40K stats don't go above 10 ever), have roll under mechanics instead of table lookups, have a simpler / more uniform method of classifying attacks and weapons, and having a unified method of presenting living units and vehicles / buildings (at least in older versions of 40K vehicles had an entirely different wounding system).
So, models would have a ballistic skill / ranged skill that they would have to roll under to hit, possibly modified by things such as range and cover. I dispensed with the idea of a S to T roll and instead weapons just had a damage stat and armor penetration stat, where units had an HP stat and armor cover / value stat. So you still get an armor save, and if you made the save your armor reduced the incoming hit by its armor value. In low tech settings most things would be 1 Damage and no AP, with armor being of Armor Value 1 and variable coverage (roll under 2 for armor consisting of just a steel cap, all the way to roll under 9 for a full suit of articulated plate). Higher tech settings like modern settings would probably give rifles 2 Damage or 1 Damage and 1 AP with most things being unarmored or wearing enough armor to give 2 Armor Value (and thus still stop rifle rounds).
It sort of works, but it's a bit clunky and doesn't seem to have the nuance I was hoping for. In particular, without the S to T roll and a minimum damage value of 1, it's hard to represent things smaller and weaker than humans.
I actually considered using abstract unit-to-unit combats using dice pools where you rolled a number of dice to see if you hit and how much damage was done, but could never come up with a satisfactory way to do infantry-to-vehicle encounters without letting guys with rifles destroy tanks or letting a single anti-armor missile wipe half of a unit out. Any fixes felt like a huge band-aid.
So, at this point I'm kind of back to square one.