I'd probably...
* Make Shiva's Crystal Blessing TP Bonus +1000 instead of +250.
* Give Garuda Hastega III, which caps magic haste.
* Make Avatar's Favor a 10 minute AoE buff that uses the gear / avatar you have out when you use it (no potency building or avatar restrictions).
There are a bunch of things I would like to do for the job (you could pretty safely double favor potencies), but I think we would be able to responsibly start using summoners as buffers or healers in a lot of content if they made these three tweaks.
Pretty much also where I'd start tweaking.
- Buff crystal blessing considerably. Maybe full potency for 1handers and 2/3 or 1/2 potency for 2handers and hand2hand
- Make Hastega II a separate buff so it stakes with Haste/II.
Crystal blessing should add enough fire power to make it attractive enough to consider it at least for 1hander DDs for BRD or RDM slot.
Making hastega it's own buff would open combos of SMN + BRD, RDM or BLU to cap party haste.
From there you can still tinker with favor changes and add buff/debuff scaling with SMN skill to bring them past the 75 era but that's just icing compared to the two changes above.