u dont have to make to many answers to planeswalkers if they are just crap
every creature is an answer to a planeswalker. i doubt that they will make "superfriends" a playable archtype in standard. it would slow down the pace of the game and usually thats somethn they at least try to avoid.
but when they will planeswalker removal, i highly think they will have planeswalkers with this ability. afterall its a war, so walkers should be able to handle walkers!
You may be underestimating the impact this will make, but you make a good point. Cards like Tormented Soul and Phantom Warrior are extremely useful against planeswalkers, and that is something we should expect for limited. I'm sure there will be some silver bullets too though.
"Ability Name" - This creature can't be blocked if attacking a planeswalker.