As of right now spirit procs from trinkets are used to calculate the amount of mana returned from the new Innervate.
So, basically, 200-300k innervates will be possible depending on how many of them you stack and what item level they are (they also give a large boost to your combat regen on top of the monster innervate).
HLG cannot remotely compete with this, but it will certainly be good early on and depending on luck. If the interaction is nerfed HLG may well end up being one of our best available trinkets, if not the best because the others will honestly be pretty lackluster at that point.
The interaction between Spirit procs and Innervate should and will eventually be removed, hopefully before 5.4 goes live. If that interaction was a problem with Rapture and Mana Tide Totem, it's a problem with Innervate.
"The total mana gained from this ability can never be less than 8% of the casting Druid's maximum mana, and the amount gained is unaffected by Mana Tide Totem."
You would think they would have just made it not interact with all short term spirit buffs when they made that change.
It probably will but, but I don't know if it should be. Druids are already mid-pack and will be bottom-pack in 5.4. This sort of interaction is the only real hope we have for being competitive. All it really does is give us as much mana as Disc and Mistweavers have had the entire expansion.
That's not terrible much to ask (if they aren't going to fix those classes insane regen... which clearly they aren't). Without this interaction all of these trinkets are pretty garbage - the CD reduction one is interesting but Resto as of last I checked is getting boned hardcore on the abilities affected and I personally HATE int procs if there's another alternative.
The mana tide exception is not that unusual as they're not wanting to make Druids reliant on Shamans for mana. You'd think some clerk at blizzard could notice the incongruous vs. the trinkets, but hey, Blizzard has messed up even more basic things before. I wouldn't mind seeing a reduction in how much Innervate benefits from spirit procs (it really is insane right now) but I think it adds a good element of play - reminds me of the good ole days of potting before an innervate.
Mushrooms in 5.3 are the only thing keeping us afloat and they're taking a pretty huge nerf in throughput and ease-of-use (how they've managed to do this is amazing).
The basic message of 5.4 PTR is reroll Disc/Mistweaver while you still have time. Tempting.