I dont think you can balance summoner spells to be more diverse

Simply because of mobility creep. It seems like all the new champions have some sort of dash, or at the very least an escape/engage while older champs have to rely on flash+whatever they feel comfortable with.

I believe a fix would be adding like a dash to boots but that would only add an extra dash/escape/engage to champs with them which would wouldn't solve anything. When looking at kass's reword(for like the millionth time) it seems like Riot wants champs to move faster, in which case why would older champs like like Nasus, Ashe etc not pick up flash.