Instead of nerfing flash, ignite, ghost, or exhaust, consider buffing the not-so-good summoner spells to the point where they can be worth choosing over the current optimal choices.
Fortify: Move the mastery to the top of the Defense mastery tree; perhaps swap it with the Revive mastery. Just not that good w/o the mastery and it's too deep in the tree now for most to take.
Rally: Allow one hero to teleport to the rally point (without the actual spell).
Heal: Add a moderate regeneration buff that lasts 20-30 seconds; enough to get a hero mostly or fully healed during laning phase.
Clarity: Add a level-scaling AP buff that lasts somewhere in the 6-10 second range.
Revive: leave it alone, too easy to make it gamebreakingly necessary.
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