How would the game break if one used a "single initiative" system?

[larsdangly]

I think all the pieces are there for a 1-stage 'I go/you go' system, but be aware you'll end up at something closer to GURPS than TFT (I think how you determine who goes first is a minor detail). It is easy to see how it would work, in broad brushstrokes, but undoing the canonical system (move first, act second) will require a ton of fine adjustments because so much of TFT's combat system is predicated on the idea that the actions you can make are limited by the movements you and your foe made. The end result will also depend strongly on how you treat 'engagement' - if you leave it as-is, game play will feel more like TFT; if you drop or change it, game play will be more like GURPS.