Aggro from another player (keep in mind animal attack will still stop your travel) shouldn't stop or interrupt teleportation to heart fire.
Why? If someone don't want to PvP, don't force him to do. A lot of sprucecaps prefer to PvE not PvP. Leave PvP for thingwall fights, meteorites - if you teleport to hf you lose objective so it won't be abused.
I think this is my the best idea ever.