There are two ways to approach this problem. The first is to add a 2D solid (if you add a white one and set the blend mode to multiply you can see through it) on the top layer, then draw your paths in 2D, then composite the 2D arrows in your scene. The second option is to use the floor and use the pen tool to draw your paths while looking at any view that gives you a good perspective on the shot. You can then copy the path, select your arrowhead and paste the path to the position of the arrowhead. This will work as long as the floor is not scaled and the position has not changed. If you have scaled the floor layer then you can use a 3D solid in the same position and orientation as the floor. It looks like things have gone wrong but they really have not. Paths that are always going to give you motion only in X and Y. This is ok. All you have to do is to add a 3D null and parent the path to the null, then rotate the null. Just make the arrowhead layer 3D, then parent the arrowhead to the null and rotate the null to match the angle of the floor (the layer you drew the path on). That's all there is to it. If the floor or the solid has been repositioned you can move the null to the floors current position and then orient the null to the floor and the motion paths will line up perfectly.
Maybe this will help. It's a short tutorial on animating a camera in 3D. You can use it to animate anything on a 3D path.
I probably would have started this project in Illustrator using a large enough artboard that I didn't have to scale up the floor to fill the frame. I would have Imported the AI file as a comp, then used the pen tool to draw paths for the players. The anchor point of each player would have been positioned at the bottom of the layer at their feet. Each mask path for each player would have been turned into a motion path for the players. I would then adjust the timing of all of the moves. The mask paths for each player could also be turned into a stroked shape layer by starting a new shape layer with the pen tool, clicking anywhere in the comp viewer and then pasting the copied mask path. You could then use trim paths to animate the path. Once all of that was done and timed I would turn all layers 3D, add a 3D null, then parent all 3D layers to the null and rotate the null -90º in X. The null would then be removed as the parent. Each player would have an expression added to point to the nulls rotation x rotation property and then they could all be stood up to stand on the floor by rotating the null back to 0º.
I hope this helps.
Thanks Gerard, I have read your suggestions thoroughly and I have a question - do you think if I put the 2D floor + the path + the arrowhead (3 layers altogether) into a new comp (and draw the path there, while checking the Main Comp in a separate window), and then use that comp as a 3D layer in the Main Comp, that could work? I am on set, shooting something else these two days, hence my "questioning" instead of trying.. But to me it sounds as a viable solution.
Anyway, you gave me a few options, I'm gonna go ahead and try them, and also thanks for the tutorial, I have checked here on set with my mobile, but definitely will get more into it back home!
Thanks a lot in advance!