I love the idea of this change, but it would need to stack with empower or GoE AND somatic bond.. just like werewolf does, or it will be useless.
This is what I had in mind. It would be a successor to Illusion: Werewolf in every way, effectively being in the same "line" as WW. The buff would stack with those effects and buffs that WW stacks with and would not stack with those effects and buffs that WW does not stack with.
I dont like the idea of rat form.. no thank you. Turn me into one of those little black constructs or one of those sanctum golems, now that would be cool.
I daresay our beloved Blink has a case of musophobia--fear of rodents! Haha. Even if the construct (called Earth Golem in model files, I believe?) were made global, would you really want that to be a form you're in for potentially hours on end? That pudgy, droopy-bellied thing that belches out unseemly noises each time it takes a hit? As stated in my rationale, I picked Giant Rat because it can draw a connection with an NPC who is often the first "hard" encounter met by level 1s who chance down his alley (probably thinking they'll find the POK book). Unlike the yard trash in North Newport, Rabb is a distinctly SOD-exclusive mob and ought to be given tribute.

(This also would fall in line with other high-end spell name conventions such as Jayla's Boon and Fist of Shiritri.)
Anyway, it's all a pipe dream at this point so why not use our imaginations at this point? Golem isn't a terrible concept, just smacks a bit too much of Giantkin's Hill Giant form for my taste.
The way to really make this work would be to have the spell grant 100% haste, and large boost to strength agility and dexterity.
Slot 1: Illusion Were Wolf
Slot 2: Increase ATK by 45 (L44) to 66 (L65)
Slot 3: Increase Attack Speed by 40%
Slot 6: Increase STR by 23 (L44) to 29 (L57)
+
Empower:
Slot 1: Increase Attack Speed by 60%
Slot 2: Increase AGI by 57
Slot 3: Increase Armor Class by Expression error: Unexpected ( operator
Slot 4: Increase Stamina Regeneration by 10
You would need to combine those two buffs, maybe add a nice lifetap proc or magic based nuke.
It would need to stack with somatic bond obviously. Not only would this open up room for another buff, and possibly make true-giant viable on raids, (if you think I have 5 buffslots for Rbow, werewolf, empower, Somatic bond and CoTP on raids you are CRAZY) Why not put all those buffs together in one big bad "Melee mode" type spell. Making the illusion not have the guards attack me every time I go to town and forget to click the buff would be a nice perk too.
While I love this idea, I already felt I was pushing my luck posing the buff effects that I did. I would gladly welcome a non-novelty proc but felt that might bolster the enchanter's DPS too much on top of the already-present ATK and STR buffs.
And lawl at the idea of having Blades, WW, Emp, SB, and COTP on all at once in a raid.
Ranger tells the raid, "SB COTP inc"
Enchanter tells the raid, "Hold on one sec"
*after a moment of intense lip-biting and contemplation, the enchanter clicks Blessing of the Combine*
Enchanter tells the raid, "Ok cast"
I dislike this idea very much.
I have a vested interest (this is my thread after all), but I also dislike the idea of a "mana battery" spell. I realize you said there was much you have yet to consider about it, Woldo, but it seems that extreme skills like Gather Mana or Mana Burn (it is still in the game, right?) could break a spell such as you suggest. Additionally, that would presumably mean both a buff on the recipient and a buff on the caster, and if a cleric has clickies, Divine Benevolence, and the works on, ensuring they receive the mana battery buff could be problematic as we only have control over our own buff count and often we do not have even that luxury in emergency situations as you intend this spell to be used in. This outsources the enchanter's efficacy even further than it already is.