Novel idea. Seen it tried before. You have some hurdles to jump.
Assault Blocks: The biggest one, by far, is I can now activate a unit, and end up assaulting another unit before it shot at or moved at all. This means that most of the time, unless you are assaulting something that was already activated, it will have little to no opportunity to act in its' own turn. So you essentially are making who goes first EVEN MORE important. The guy who goes first in Game Turn 2 could shut down most of the other guys' shooting phase with assaults, this would make assault armies IMMENSELY powerful when they get first activations on a turn...it would just go "Assault army activates, gets a charge off...opponent activates another unit, shoots something...assault army activates another unit and gets another charge off" that can quickly snowball.
Time and Turn order: This takes immensely longer than the standard turn structure, so you're going to have to rewrite the way the game goes on for X turns and such.
Balance Issues: There are things in the game that affect a unit under the premise that they have not yet done anything (for example, Blessings/Maledictions). These things are too numerous to even begin to list, honestly. All of them will need to be addressed as many abilities or powers will become useless to use on a unit that already activated (i.e. activating to put a blessing on your own unit becomes stupid unless the enemy isn't going first).
Tracking: You are going to end up with a board full of confusion just because of the number of units and models there can be. There needs to be a dedicated way to say who acted and who didn't (like some kind of marker that is very visually obvious and gets flipped). Dice will get out of control fast.
Synergy Destroyed: There are times in every game I have witnessed where there are strategies employed like trying to hit a unit and provide a beneficial scenario for another unit to finish it off, etc. A lot of those are going to be ruined (think things like "Hey those Long Fangs already shot so now the rest of my army is safe from them" type stuff). This is actually less tactical, there is no reason for me to be careful as a player, you just put off activating certain units until the other guys' main threat to them is tied up).
It can work if these issues are addressed but it will take a lot more work than 5 sentences of notes.