If you nerf, some people will lose interest, this is inevitable. Also, the "supermyths" will always be there and never be reachable.
If you don't nerf, more and more areas will have to open up to allow for more and more supermyths.
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Lets be the devils advocate, a solution that can solve this, but will probably be hardest to implement (thank the Gods I'm not the one who would have to code this

- Staged nerf... From approx rising hero start up to, say middle in the range between legend and myth, reduce the power from current 270% down to the intended 100%.
- Reducing recovery bonus from it's current (lets just call it 100% for eez) 100% down to zero over a span from veteran to titan/champion area.
- Reduce the benefits of quickness in General or just make it much harder to get a high level of haste, for example reduce all haste spells potency, no item giving more than a certain low level of haste, (even) smaller cumulative effects from multiple objects, make highest haste imbuement the best speed-increasing boost you can get, including spells, and still not make it too good.
Results would be that growing to mid-sized legend would go slower, but still give you a little bonus in damage up to that point, but gradually, letting those who are growing still can gain some size, but it will flatten out, and has to be done in such a way that a smaller players special should not outdo a larger players special in damage from same guild of course, but this is left up to the coders to solve

Secondary result would be that being careful with dying becomes more and more important the larger you get, as well as there being a chance of reducing the size of supermyths, should the players wish to take them down

Before anyone bitch too much about how this won't affect me, well, I recently had two deaths, which would be a lot harder to recover from if this is implemented during the two weeks I'm now being unable to keep up grinding to a high level, so there is a chance it will affect me, and being as close to the top of the reduction-span on the specials, that would affect me as well, and speed is one of my favoured things, so yea, again - full effect.
It's an idea, it might be the best solution, but it will also allow for the game to even itself out over time (perhaps a very long time, but..).
Tulasi Waylan the White