I was recently in a discussion about how 8th Ed 40k is unusual among 40k in that although the general turn structure is "I go, you go", the Assault Phase replaces Initiative with a pseudo-alternating activation system. In theory, it shouldn't be too much of a stretch to make the game fully "activation turn-based", though as usual, there are assorted hurdles to handle.
This is an super-early draft. It doesn't have Psyker rules implemented yet, and it could be theoretically broken by extreme MSU (lots of solo Acolytes), but the intent is that it would serve to mitigate alphastrike-play, be relatively simple to learn, yet model fluid tactics and allow for sneaky play. These rules are pretty edition-agnostic, though the removal of Initiative makes it easier to plug into the 8e Assault Phase rules. As usual, comments and criticism are appreciated:
Command Points: Command Points are more restrictive than normal 8e, but they replenish at the start of the turn. Rather than being used for passing Battleshock, rerolls, or a "counter-fight", they're used for coordinating your forces.
Actions: A unit gets a max of 2 actions per turn, chosen from the following options. A unit may move twice, but not take any other action twice. I might change this rule to be "only one of each action", or allow "double actions", depending on playtests.
* Move: As in normal 40k, without the option to advance. You may not move within 1" of an enemy unit unless you intend to end your movement within 1" of that unit (alternately, you may move within 1" but this also brings up Free Strikes) If your unit started its activation in melee, moving provokes automatic hits from all enemy models in b2b with your unit ("Free Strikes"), but you may still shoot after withdrawing. At the end of this activation, any models in this unit in B2B with the enemy may make melee attacks. (Alternately, the unit may make a round of melee attacks against any single unit it moves within 1" of, so you could do ride-by lance attacks, etc).
* Attack: Your unit may make a 3" move. If the unit is not in melee, this move is without restriction. If in melee, this move is used to pile in, as described in the 8e assault phase rules. Assault Weapons can be used in melee if not used previously. You can split attacks between 2 melee weapons (at least one attack per melee weapon), use one melee weapon and one pistol, or two pistols.
* Defend: A unit that defends gains a defense token for the turn. A unit is -1 to-hit for each defense token it has. Defend replaces Smoke, Jink, Go to Ground, etc. and is a general catchall for protective measures.
* Some characters get noncombat actions they can take in lieu of attacking.
Activations: You may activate a unit that has not taken any actions, or spend a command point to activate a unit that has only taken one action. The activated unit may take up to two actions.
Interrupt: Immediately after an activated unit declares an action but before executing it, the opponent may declare a unit will attempt to Interrupt. This costs nothing if the unit took no actions, and 1 CP if the unit had previously taken an action. An Interrupting unit may immediately take an action, and the activated unit resumes its original action.
Ending an Activation: Once a player is done performing actions with an Activated unit, the next player Activates a unit and can take up to two actions. However, the first player can spend a Command Point to activate a second unit before ceding the Initiative back to the next player.
Reserves: Reserves come in by activation. Bringing in a unit from Reserves requires spending a Command Point, in addition to any normal CP expenditure.
Note that this system has no random charge distances, and transport disembarking is by activation rather than at the start of the movement phase (so Rhino Rushes are in). Inversely, the system has no Overwatch, at least unless you're willing use Command Points on activating "half action" units in response to your opponent.
The system would have a few oddities/tweaks to represent old-school rules, hypothetical examples including:
* Dark Angels do not need to spend a CP to interrupt with a unit that moved, if that unit is attempting to shoot a unit that is declaring melee attacks.
* Dark Eldar have 6 - (Turn Number) CP per turn. They need to get in, accomplish their objective, then get out!
* Berzerkers can take two Attack Actions, if they're both with melee weapons.
The rest would be fine-tuning, and getting tests in. Feedback is appreciated of course.