I bought this up when I saw your thread for it in Forbidden Lands. Guess here works too.
If I might make a suggestion.
Instead of removing action economy entirely you should open it up with soft caps instead of the games current hard caps.
The game as written has hard caps. You get one slow action and one fast action or 2 fast actions and that is it. You are actually INCAPABLE of doing anything else.
Instead, you get 1 slow and 1 fast (or 2 fast). Each additional slow action you attempt comes with a cumulative -2 difficulty penalty and each additional fast comes with a -1 difficulty penalty.
So for example: I want to move forward and attack twice!
Move action (fast), 1rst attack (Slow), 2nd attack (slow -2 skill dice).
The opponent attacks you back. You attempt to parry (fast -3 skill dice (This turn you take the -2 from the second slow action and a -1 from the second fast action)).
Order of operations would be important. "I want to move twice and attack!" Move (fast), move (fast), attack (slow -2 as your first additional action).
(This part is in reference to Forbidden Lands but will probably apply to Alien as well)
As far as I can tell this would not negatively impact any of the Talents currently in the game as those talents give you additional actions for free or at no penalty for spending a WP. So those would still be super useful for getting more actions without penalty while still letting players attempt to defend themselves.
You don't NEED to spend WP OR suffer damage, but with the reduction of skill dice the chance of having to Push or spend WP for other perks is significantly increased and the more someone attacks you the more likely you are to fail/take damage anyway. Penalties continue to accumulate until the beginning of your next turn at which point they reset to zero. It's more interesting to give the players the choice with the risk and gamble then it is to draw a hard line in the sand or give them no other option but to pay.