It is very likely that the next iPad mini will get the feature, because IMHO it is very likely the iPad mini will get the A9 processor this fall.
The iPads are due a chassis redesign this time around according to the normal cycles and I think they will be getting the iPhone 6/6S design treatment. The iPad mini was the first chamferred edge design iDevice, followed by the iPhone 5, and then the iPad Air. Pretty unprecedented for an iDevice to go three updates without a chassis redesign, actually - can't recall another, not even the iPod touches have ever gotten that neglected, IIRC.
Now the iPads will get the rounded curved glass edges, and see a thinness bump along with it. The A9 will have 2GB on the chip, and it is likely that both the iPad Air and iPad mini will use this chip. If the iPad Pro does in fact get released, which I have a gut feeling is actually doubtful - given my interpretation of how WWDC has gone and what I've read about iOS 9 so far - then it will probably be the only one to get get the 'X' variant with extra CPU/GPU cores.
If Apple can make the A9 a better performer than the A8X while making it more efficient (remember it will be on the new 14nm process as well) then they don't need to make an 'X' variant this time around and can save even more on scaling up use of the A9 across all the 2015 iDevice range - iPhone 6S/6S Plus, iPad Air "3", iPad min "4". Maybe upclock the CPU on the iPad Air version due to better thermals with the larger chassis. This year, Apple is going to make one chip, the A9, and put all of its new device capacity into making it.
I just think that there was a particular reason (that we are obviously not party to) Apple did what they did to the iPad mini 3, and that parity will be restored this year. I actually have a hunch that it was the A8X itself that was to blame. With the A8's all going to the iPhones as fast as they could possibly make them, and using remaining capacity resources towards producing a lot of A8X's for iPad Airs that the iPad mini couldn't use for packaging or thermal reasons, it may have been forced on them to continue using the A7 for the iPad mini. Remember, according to usage data, the mini accounts for only about 5-7% of iPad sales. But even given that, I don't think Apple are going to have the mini be a lower end iPad long term. Like I said, I am of the strong belief that 2014 was a blip rather than an indication of future direction.
It will also simplify things for them to keep iOS versions and feature sets as simple as possible. As long as the iPad mini and iPad Air have the exact same screen resolutions, I think Apple would like to keep them on feature parity as well. They already have a half iPhone/half iPad Frankenstein feature set of iOS for the iPhone 6 Plus, and this latest further feature set fragmentation with the iPads can't be what they are aiming for. When both have the same RAM in addition to CPU/GPU, they will both have the same feature sets.
announced the split-screen only for the Air 2. It would make sense to put 2GB for that reason. They'll probably put a 1.4GHz A8X with memory modules inside the chip, kind of the same way they differentiated the A7 in the iPad Air and the A7 in the iPhone 5S/iPad Mini 2/3, but seeing the X would give me an OCD attack since the Mini has never had an X chip.