This would alter the gameplay alot.
But i'm for this change.
Just add some random agression value based on race and mission and ufo weapons compared to x-com weapons. Possibly make this also influenced by interceptor approach (more cautious approach makes lesser chance ufo would turn agressive).
Then ufo would decide if it wants to fight back or not.
Once you chose any option to approach the UFO, if UFO chose to be agresive, even if you choose to disengage or go to cautious mode, if its faster UFO would maintain distance enough to fire its own weapons +- some percent (indicating there s a dogfight going on).