Regarding movement, turns and time flow in PP: is a bit of a tricky thing, really. As PP seems to take some design decisions out of FXcom, we can currently treat their core approach to turns in a similar manner. And the thing is, while both games are clearly turn-based, they both are pretending that time is not stopped between actions and is actually always flowing.
Think about it: combatants have idle animation, they move around, peek from cover etc. It’s never explicitly stated and in FXCom when we have some kind of game mechanics which tracks “time”, this time is in game turns and is never progress unless a turn is taken. But the overall “presentation” of the battlefield between actions looks more like a bunch of combatants dug into cover and waiting for an opportunity to jump back into action rather than people who are “frozen in time”. Again, it is never addressed explicitly and mechanically is it all turn-based but one thing FXCom never touches is the difference between being in motion and being stationary at the end of the turn.
Here is an example from FXcom: a soldier is ordered to do a 1 action move from cover into an open ground. The turn is then ended. Now, it is quite clear that during his turn, he was moving. But if you want to determine his state during a subsequent enemy turn, will that solder be considered moving or stationary? What if he used both movement actions? What if he ended up in a cover? How about his next turn: if he moves again, is it a new dash or he just continues his run he started his previous turn?
All these ambiguities are unavoidable for a turn-based game as the whole idea of truly turn-based game mechanics is to alter the way time works in the game and turn the flow of time into a set of arbitrary rules which are more consistent and easy to deal with at a cost of straying away from how time works in real life. It’s not a bad thing per se but it has it’s limits and some thing just cannot be expressed as elegantly through turn-based logic. It is much easier to deal with such things with a real-time-with-pause approach due to it’s more natural progression of time and simultaneous action execution(XCom and UFO games are good example here) but as PP is strictly turn-based, we will have to deal with what we have.
So back to the question at hand: do we need any kind of evasion mechanic and if so, how to implement them? I think it is an interesting and realistic concept(moving target is harder to hit). I would like to see it as variable value, which increases with distance traveled in a turn. But in order to make it work with the rest of the game mechanics we need to make some compromises.
Firstly, there should be a hard cap on how much evasion can be gained by a combatant by moving around, to avoid extreme evasion gains via exploitation of various movement aids and movement-related skills. I expect max possible evasion to be close to the defense provided by a low cover. After all, on the move or not, you are still a human running in the open so you are not that hard to hit.
Another important thing is, movement-based evasion should be viewpoint-dependent. What this means is, it should be calculated based on angular distance the target covered relative to shooter’s position. The closer the line between a target origin and it’s destination is to being parallel to the line between shooter’s position and a point in the middle of the target’s avg trajectory, the smaller the evasion bonus is. So after calculating the evasion based on traveled distance, it will be further reduced depending on how parallel the trajectory was to shooter’s LoS(see fig 1 below).
In other words, if you run towards the shooter(trajectory is parallel to shooter’s LoS), you gain no evasion as from shooter’s perspective, your silhouette is not really moving at all. If, however, you run across shooter’s view, (parallel to shooter’s LoS) you will gain a lot of evasion as shooter will have to track your movement across his viewpoint. For reaction shots while a soldier is on the move, currently held evasion value will be used, i.e. if an enemy overwatch is triggered while a soldier is half-way to his destination, the distance between his origin and his current position, not his destination should be used.
Due to simulated ballistics of PP, evasion bonus will have to affect the simulated deviation of shots. While mid-action shooting(due to overwatch and similar abilities) can simply take that into account, manual aiming should have a way to conveying the reduced chance to hit to the player. I think the best way of displaying it is to elongate the aim circle along the movement axis, clearly displaying the lowered chances to hit the target(fig 2).
Evasion bonus must also be reset to 0 when an action is performed a if movement is ended in a cover. So for multiple movements between covers during one turn, or movements separated by actions, only the last uninterrupted movement sequence(i.e. with no actions in between movements) will be used to calculate the end of turn evasion bonus(provided, of course, the movement is the last action in a turn and the soldier ended up being in the open).
This way we prevent cheesy tactics like jogging between two covers to keep evasion bonus while still enjoying the benefit of cover and avoid the complications of dealing with performing actions or shooting on the move vs stopping for actions: all actions which require APs should be considered to be performed while stationary.