Mages are the only calling that has two pet trees. This presents a bit of situation when it comes to the souls that those souls can partner with and the synergy that each soul offers the calling. Keep in mind, these suggestions aren't theorycrafted; they're more so to spur along the creative process and create a ground to brainstorm.
To me, Elementalist uses their pets to increase their own power while Necromancers instead work to increase the power of their pets. Due to this, however, neither soul works well (see: hardly at all) with the other, which is rather odd considering their both pet trees. These talent changes, I think, would allow both Elementalist and Necromancers more options, giving those players who actually enjoy playing with a pet more ways to be creative.
Crystalline Missiles, Elemental Link and Ashen Armor should include all Mage pets, instead of just Elementals in order to promote Necromancer synergy.
Elemental Forces should affect both the Mage and any summoned pets, including things like Split Personality, Empty the Crypts or Swarming Flames. The change to Spotter's Order crippled specs like the Zoomancer (Necro/Dom) which I know several people found enjoyable. This would mimic that while at the same time limiting the duration due to charge consumption.
Weathered should increase the health of all Mage pets by 50%, and all Elemental pets by an additional 1-2% per point spent in the Elementalist tree, encouraging Necromancer synergy.
Sacrifice should be changed so that the summoned Elemental gives a particular effect when sacrificed, along with being healed for 25% (i.e. Earth Elemental summons a shield, Air Elemental gives a spellpower boost, Water Elemental gives an immunity to crowd control effects). As it stands, an Elementalist has very little reason to Sacrifice their pet, especially while soloing.
Fiery Assault currently is a weak AoE that consumes charge, yet has a long cooldown. Perhaps it could be made better if its target limit were removed, making it a unique spell in the Elementalist's arsenal, especially given the soul's weak AoE capabilities.
Synergize is an excellent canidate for a boost in synergy, and would probably be amazing if it switched places with Sacrifice and changed to allow any permanent pet to gain the buff.
Tempest could be changed to better reflect how an Elementalist works. Alongside the increase in crit damage, the same affect could be applied to the pet. Both the pet and the Mage's crit damage could be increased by an additional 1% per point in the Elementalist tree over 36. This would really work to boost the Elementalist's single target, while the numbers could be adjusted to prevent its simple rotation from being the absolute best. As the game progresses, it would become more and more viable a soul, but its lack of mobile damage would mean that movement fights would require another choice.
Death's Ally should be slightly changed, increasing the Mage's death damage by 5% and any pet's damage by the same amount, instead of limiting it strickly to death damage.
Consumption should be changed into a two point talent, giving 5-10% mana regen.
Greater Consumption should be reduced to 1 point changed to have a chance to remove an additional buff.
Essence Link should allow any pet to proc the heal instead of just undead pets.
Donate Bones should be changed. It should be a triggered explosion that spreads Deathly Calling in addition to its current effects.
Life Bane should switch places with Master Chirurgeon.
Master Chirurgeon, like Weathered, should increase the health of all Mage pets by 50% and all Necromancer pets by an additional amount per point spent. The two talents should be allowed to stack.
I'm not stating that all, or even most of these changes are required. I most certainly don't want either soul to become the absolute go-to soul for DPS. What I do want is for players to be able to choose and be able to have more options to have fun. I believe that fun starts first in how the class plays and interacts with itself and others, and the numbers follow behind it. No one wants a top-DPS spec that just presses one button.