I tell you that… Overall most Mercy players would be more than okay with a cast time on Mass Res and without invencibility. The thing is… just doing that is, making the game better and thats the thing everyone wants, but it would also make Mercy very weak, so an E has to come with it and in best case she is still able to move during the cast, forcing better timing. Some other stuff on top of that and you have a very well balanced hero.
I suggest that E ability (At 3.3) and change to Resurrect (3.1):