Simple solution: PvP armor perks
When you consider that year 2 Trials and Iron Banner gear comes with PvP centric perks (for example: Angel of Mercy or Second Wind) maybe this could be the answer.
So let's say instead of altering Touch of Flame or Viking Funeral in any way, there could be armor perks that
-negate solar DoT
-immediately start shield regeneration after the effects of DoT burns expire (Thorn included)
-increase armor while under the effect of solar DoT
-reduce tracking of AoE grenades
So on and so forth. It wouldn't have to be limited to Sunsinger counterplay either. There could be perks to counter wombo combo, shoulder charge, suppression grenades, tripmines, etc, etc.
Discuss.
Edit: So everyone seems to think I'm suggesting this as a way to only nerf Sunsingers. That is not at all the case. So here's some more food for thought.
Say you pick up a Crucible chest piece and it has 2 perks on it. One that reduces non precision shotgun damage and the other that grants max armor while under the effect of solar DoT.
Then Shot Package could be reintroduced to the perk table and there would be a meaningful trade-off to trying to counter firebolt grenades.
Obviously, it may not be that simple and there may be things I'm not thinking of but this is the general idea I'm trying to get across.
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Except for one thing. Bungie says firebolts are fine (which i agree with) but apparently everything else fun to use is OP.