I think for mo and krill to ult, an indicator of Krill's gun to charge up yellow for like .5s before making contact with the enemy hero. I find it that if he's in your face dealing damage to you (which they like to do) an indicator on any form of counterplay be nice
You can tell when a hero's ult is up by the white circle under the heroes portrait at the top of the screen. If a mo and krill walk or "dig" up to you, its healthy to assume that the guy wants to ult you. Setting aside his burrow ability, its worth noting mo and krill has to walk up to you to ult