I think there is a problem with getting people to mark targets, which hampers the 3As, armor, air and arty. If people were granted some % of the XP someone hitting one of their target markers gets, maybe there would be more targets marked. Right now getting people to mark a target is almost impossible. If you are using air, or another of the 3As you just hit either an Objective or a random spot hoping there is an enemy there.
Yes, that would be great. Unfortunately the marking system now is works like: “You can push V button to make re marker anywhere” but, if the marking would works like in H&G etc., when you can mark target only if you aim directly toward the vehilce/soldier etc. and its marks the last position of the target.
(Important to highlight this: Don’t make marks system to behavior like in BF, when the ‘red marking tag’ follows the moving target, like in BF…" Also plane spoting not works well I think…
OR, when you push a mark around/near the target, and someone kills the target, you can get XP and point for marking. Obvious, if you just push random mark/or too far from Tank/Infantry target, than you dont get nothing.
I don’t have much problem with the marking system as it is. Very simple for sure, so it could be improved a lot, but those markers also instruct your AI where to focus so there is a reason you can set them anywhere.
For the XP bonus: there should be a distance threshold - if tank moves away 20 m before first attacked, you should not receive XP. Or should you?
…Now, I see some hidden algorithmic complexities in that single sentence. Good luck implementing it so that it feels right with multiple people marking same thing, member of squad leaving marker and other member occupying its place, tank being almost lethally hurt on marker then moving on etc. 
Im cool with the marking system now, actually, but I write multi- of my ideas for that reason, if the Devs wanted to improve it in the future.
Maybe some direct mark would be the best option. When someone alrady marked the target, the other player cannot anymore, only just confirm it. There is just one problem with a current marking system. When you mark a Tank for example, and push V again a little bit next to it, its set 2 same red tank icon next to each other or 3 of them…would be enough ony 1 icon per vehicle or per person.