It would be cool if more keystone runes had a boots active like Predator.

Since you can only have one keystone they'd never overlap. I think a lot of the runes would have more depth if they had this. Here are some ideas.

Electrocute - Activate to make your next Electrocute within 15s deal double damage. 120s cooldown.

Summon Aery - Activate to send Aery to shield target allied unit. 60s cooldown.

Unsealed Spellbook - Replace the 30% summoner spell CDR with: Your boots can be activated to use a copy of the last summoner spell you replaced. Can only be used once per replacement.

Aftershock - Replace the requirement to immobilize an enemy with a boots active so any champ can use it.