I really don't see Mattel doing the Proton Pack. Sorry, but I don't. The Trap, Goggles...maybe. But not the Proton Pack. That PKE is bloody close to the real deal, a 'toy' Proton Pack that is as close to the real deal as the PKE is, will cost a lot of money to get right, and would probably retail for a good couple of hundred dollars. True that's cheaper than building one.
But then they run into the same issues MR had.
1) If they issue it with straps/ALICE frame, it's wearable out of the box. If it's wearable, it's a COSTUME accessory, and then that infringes on Rubies' licence (I wonder why Rubies haven't yet come up with a 'deluxe' Proton Pack to replace the inflatable one that comes with the costume?). In all likelihood, Mattel would have to issue a Proton Pack with no straps or frame but leave it open for those things to be added, like MR would have.
2) Reaction from GB prop builders. I checked out the reaction to MR's announcement that they would do GB props and to me, it seemed mostly negative. Mattel have lucked out on the PKE because it's not as widely built by people due to cost, so much money for such a little piece...but the Proton Pack is the core bit which people don't mind paying a lot to make. So you're bound to get a level of negative reaction similar to when MR said they would do a Proton Pack. You'll get the concern over legal ramifications for people who make and sell parts (it's a livelihood for some people, and the reaction over the MR proposals suggests some of them think they have an automatic entitlement to that livelihood), and you'll get the people who have strived hard to build an accurate pack and who will feel like their work is undermined by an 'OFFICIALLY LICENCED REPLICA'.