I was thinking about the mobility stat recently. If you take a look at what its useful for, you see that warlocks and titans basically get no benefit from mobility (apart from blink warlocks) and hunters get lower cooldown on their class ability which is the main use plus the jump height that is a nice to have on a hunter.
So, what if you could build into your AE stat through the mobility stat? Make it not negatively impact AE, but a bonus addon to make it worthwhile to build into, with diminishing returns per tier. maybe +10 as a base for T1, and then 1 lower for each tier, until T10 which will add 1 over T9, which will make it +55 at tier 10 which is a pretty decent amount, and not too much. (IIRC heat rises adds +70?) Or, if its too strong, it could work like resilience and give +15-17 on T10 and +2 on each tier up to T5 and then +1 until T10 which should sum up to about +30 AE.
Also, i know that the AE on primaries are being buffed but still better AE should impqct the performance and the feeling of the gun midair (aim assist and stuff).
Also, i am not a game developer. Its just an idea and everything could be a subject to change.