So the blooming aspect of mushrooms is a very thematic mechanic that Blizzard obviously wants to keep, but is stumbling about trying to fix. So what are thoughts on handling some rather clunky mechanics to streamline it a bit, as well as making a reason for them to be placed down in the first place?
First off, I think the rejuv overhealing converting into extra healing power for the bloom is pretty clever. It offers the choice of playing normally, or sinking mana for a big desired big bloom. The problem with the current method is that the saved up overhealing is currently saved in each mushroom which makes the mushrooms' current mechanic far too unfriendly for basically every kind of raid encounter Blizzard wants to actually make. Instead why don't we have the stored up healing saved on the druid, the druid saves up extra overhealing up to 1/3 of their total health, and then once 1/3 is saved up a charge is built a second charge starts filling and after 2/3 a third charge starts. Mechanically this keeps the same function, but the charges are never consumed until a bloom is used, then a mushroom will consume a charge, three charges means three super blooms, one charge one super bloom and two regular blooms. The mushroom(s) that will use the big bloom(s) can be set up with the bigger models, so you at least know when you're placing stuff down what is going where, rather than having a random roulette when you push the bloom button. This would also enable micro for filling up just one mushroom quickly and using it when needed, and possibly make it so resto druids can “save up” bloom charges to use one at a time when needed.
Now while the quality of life change to the mushroom overheal storage system will be nice, it won't fix some of the issues that arise from having absorb happy healing mechanics in raids that quite simply make it very difficult for any real healing to get anything done. To top that off, a couple of the druid skills that should be used for certain tasks aren't simply because of a utility and risk vs reward issue. So what if we also set it up so that way every time a nourish is cast the mushrooms let out a small aoe heal, something with maybe slightly more or equivalent oomph to a healing rain tick. Similar diminishing returns for stacked up players, and discourages only placing down mushroom all the time in light of the earlier mechanic change. Choices are given, and a toolbelt that had more weight thrown at it the past handful of years, without anymore really useful tools being added, finally gets a couple extra useful tools.
The goal isn't to overpower druid healing and make them “the thing to stack” on heroic encounters, but simply to make druids functional in the real raid environment. In a vacuum or environment without priests and paladins, resto druids can probably do just fine, and maybe thensome, but since such a place doesn't exist things need to be adjusted so things work in the real raid environment.
Anyways, those are just my thoughts on the matter, I am one of the few that actually like the mushrooms, I thought they were pretty fun and cool to use when they first were introduced back in Cata's beta. I had fun with them when I was raiding as balance, and I was happy to hear about them being added to resto's toolkit as a “burst” healing mechanic. They have since then turned out as a little lackluster, and so yeah, maybe this will fix the issue they were supposed to fix but didn't.