The pop bumper, as the base of the bash toy, would give the bash toy more life.  In the same way the magnet successfully adds life to TWD LE/Premium's WW, Xmen's wolverine and Ironman's Ironmonger.  What I don't like about the TWD Pro is that the ball just bounces off the WW without any action - the WW absorbs  the energy of the ball, preventing it from being a 'feel good' shot.

Using WW as a toper for a bumper could have been cheap and would have added heap of fun, but I'm sure it never came to Stern's minds.  I expect it could have been mechanically designed to also spill its guts when hit.

...maybe on a future game?

With your excellent idea in mind, I will do some investigating and some creative thinking and see if the Well Walker or even a smaller version or different zombies can be placed and attached on top of the pop bumpers to move and also add visual excitement.

An example of this has been done back in 1991 with my Teenage Mutant Ninja Turtles machine with the placement of the TMNT crew sitting on top of the pop bumpers. Simple but very effective.