Instant Spells targeting Planeswalkers
There is a buffer when a Planeswalker is cast that Instant spells cannot be cast. As soon as a Planeswalker is cast, Instants should be able to be used before a Planeswalker can activate an ability. Currently, once a PW is cast, there is a moment where the PW can use an ability before an Instant can be cast. Instants are INSTANTANEOUS and should be allowed to be cast at ANY moment. This would allow for people to do lethal damage/kill a PW before an ability can be used.

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BeanMan#72373 commented
Your assertion is false. You do not have priority directly after a planeswalker spell resolves.
Relevant rules:
117.1a A player may cast an instant spell any time they have priority. A player may cast a
noninstant spell during their main phase any time they have priority and the stack is empty117.3b The active player receives priority after a spell or ability (other than a mana ability) resolves.