It seems odd to me that such powerful global effects as Spirits are confined to one Profession. Other professions should have access to similar effects.
As a tentative example of such an ability for a Mesmer:
Seal of Forbiddance
10 Energy, 5 sec casting time, 30 sec recharge
Create a level x Seal. Within its range Spirits take y additional energy and z additional seconds to summon.
Think of a Seal as being something like a Spirit with a different graphic. I don't specify values x,y,z because I'm not sure what levels they would need to be set at to work appropriately. Inclusion of such a skill could provide a balance against Nature's Renewal without completely hosing it as a power.
Other variations for the other professions could be devised as well. Elemental Seals that increase the powers of their respective elements for Elementalists. Death Spirits that effect the properties of Minions for Necromancers. And so forth.
There have been been many posts about Spirit spamming and ways to balance it. I think the best way to balance skills that affect the entire playfield is to include similar skills that counter their effects, or provide compensatory effects of their own, thus opening up new areas of strategy, rather than just slapping a nerf down.
I don't think that this idea is anything new, but I wonder if perhaps A-net is unwilling to balance the current powers directly because they are intending to balance them later by the addition of more powers?
E

*pales visibly* There there, little Mesmer....mother won't make you use such a nasty skill.