I know how to nerf Flash to be less effective but still keep it's viability.

Buff Revive, so that it's not a spell that trolls use for kicks on builds that's about as effective as using ghost while in base. Consider reworking it's effectiveness so that it applies a debuff but has a shorter cooldown and can actually be used competitively for competent players, since most of the other summoner spells are jokes anyways.