The idea for this addon mainly came to me as the question arose whether it's more efficient to dispel a dot or heal through it, but I figured it'd be just a short step to expand it a bit further. Here's the gist of it:
The addon would have two functionalities. First, it could give an estimate of the damage that a dot will cause. This could be implemented as a Grid module, so to say the inverse of the incoming heals estimator. The second part of the addon would be the average/median damage/second estimator over the last x seconds (possibly factoring in HoT applications). I'd think that this second part would be useful for raid main tank healers, particularly druids, so that they can apply just enough heals to keep the MT topped off, but limit overhealing.