So I would like to share with you some tweeks I've made to combat in ACKS, and of course I'm open to suggestions and scrutiny!
In my games I use positive AC and attack rolls (just like D&D5E), I also use critical hits as double damage OR an automatic success on a combat manouver, I also treat rolls of 1 as critical miss, rolling on a Fumble Table of my doing. Double damage works by doubling the damage after rolling and after adding modifiers, so usually the damage output is greater than normally it would be.
But the main difference to ACKS standard comes to the advantages of the many types of weapons that the system offers to us. In my games there are bonuses and/or penalties depending on the weapon you are using.
Large two-handed weapons = -1 Initiative / +2 to damage.
Shield = you can destroy your shield to avoid damage from one source (you can decide to use this after the damage roll). Magic shields are able to do that without being destroyed a number of times per day equal to their magic bonuses, so a +1 shield would do that once per day, a +2 twice and so on. Similarly magic weapons usually deny this abilities by a number equal to their own magic bonus (so yeah, you can't use a common shield to ignore the damage from a magic sword).
Small weapon = +1 initiative.
One weapon + another hand free = You gain +1 on attack rolls or +1 to your AC against melee attacks, decided on the Initiative phase. May use free hand to interact with stuff.
Two weapon fighting = If you hit you can roll the damage for both weapons and take the higher result. You can choose use 1 medium weapon and 1 small (so 1d6/1d4) or 2 small weapons (+1 Initiative, 1d4/1d4).
Ranged weapons in short range = +1 attack roll and +1 damage.
My initiative works a little differently with NPCs. I divide my NPCs in "Individuals" for bosses and characters of some importance, and they are subjected to the above rules and benefits... and "Groups" for the unnamed NPCs that roll initiative collectively... they can't benefit from these modifiers. So if my players are facing a group of Orcs and their mystic leader I would roll one initiative for the Orcs as a group and one roll for the singular mystic leader, who could benefit from using a different set of weapons.