I don’t find Tell or Vela or Grave overpowered by themselves, but they make an amazing team. Perhaps instead of working so hard to get an exact nerf that doesn’t weaken a hero too much but makes it strong enough for people to care, there should be limits to the power of a team at each trophy level (or some other arbitrary level).
Make sure (?) a hero’s power does a decent job of indicating value/strength—for example, strong hotms with lots of talents are higher than season one 5 stars with no talents—and limit the balance so that players need to make decisions to choose a team within the limit (eg, do i use three Top heroes in the middle of my line and weaken the wings or have a generally strong line with only a top tank). I think giving players a reason to choose a team other than pick all the strongest would keep things more interesting and lead to more fun raiding and war fighting.
You could use analytics to give heroes that are the most used a higher value and tweak that value automatically in each release.