Frankly, if I were to even consider rebalancing non-Flash summoner spells, I'd create "overpowered" versions of all of them first, and then tone down from there.
Barrier: Grant target damage immunity for 1.5s
Heal: Heal for X + Y% of their Max Health to all nearby allies. Y starts at 25% and is reduced by 5% for each target in addition to yourself.
Ghost: Unitwalking, +20% MS, +40% AS, +30% uncapped CDR for 7s
Cleanse: Remove all CC and grant CC immunity for 2.5s
Teleport: 2 seconds channel time, 180 second CD
Exhaust: Stun for 2s, short range skillshot, bypasses non-Champion units
Ignite: Instantaneous damage + vision for 5s
Smite: Quadruple gold for Smite kills
Clarity: Refresh all non-Ult spells
Revive: 300s CD
Flash is always good. Whether on offense or defense, at the start of a fight or at the end, whether teammates are alive or dead, Flash never stops being useful. Therefore, any narrower Summoner spell, to even have a chance of being "better" than Flash, must be so good that you're willing to trade Flash's versatility for it. They need to be overpowered in their niche, which none of the current spells are. They're all just plain weaker than Flash, regardless of mitigating circumstances.