Quote from Mark RosewaterThere’s a famous quote by car inventor Henry Ford (which he might not have ever actually said): “If I had asked people what they wanted, they would have said faster horses.” People can only see the future through the lens of the past.
The same is true with planeswalkers. The most we’ve ever put in a set is 6. 36 is six times that. The most common number of planeswalkers is 3. 36 is twelve times that. Sets sometimes have 2. 36 is eighteen times that.
War of the Spark, by definition, can’t just be planeswalkers the way we’ve always done them. The challenge of the design was rethinking a lot of things including what tools were available to us.
War of the Spark is not going to be planeswalker status quo, but then innovation seldom is.
Some speculations I have seen about what they can do with rare/uncommon planeswalkers that breaks with previous design is having planeswalkers with no + ability (has already been done with Sarkhan the Mad), planeswalkers with static ability, and very narrow planeswalkers, or a combination of all of these.
EDIT: Rosewater has also talked about how WAR was made possible due to new collation technology, which pretty much rules out the "planeswalker slot" solution, as that is something they could have done since forever.