Couple ideas

[Sir Baron]

We really need a good way to tell activity
I will see shielders look as if they are going AFK after just shielding for a wave
Sometimes I will see it before the wave is even over

I will say if you base it of activity you have to find some way to give shielders there reward
Same with a healer

I myself can't think of a way to do that but I bet there is some how