I don't think they would do a full redesign for the 10.5" model. It will certainly be updated, which is necessary, but I think they'll let the iPhone ride with the new design for a bit before porting it over. 10.5" is enough to get some people to buy it—especially if it has the same workspace as the iPad Pro 12.9" at a higher PPI—and then updating it in another year with the new design would get some more people on board. What I don't often hear people talking about is the software, and I think ipad software will see bigger changes at WWDC this year. But is that enough to hold on a launch? I think the original iPad shipped with iOS 3.2 or something weird, so it's not impossible that they would update the hardware and then preview the new software at WWDC for launch in the autumn. But if they do that, would they just wait to announce it at WWDC? Maybe show full Xcode running on the iPad Pro?
As for hardware features, I'm hoping for more RAM on the smaller model, graphics capabilities approaching newer Tegra chips from Nvidia, even better Pencil capabilities, 3D Touch, a solid state home button (it really is better after using the iPhone 7 and weird when I push the mushy button on my iPad Air 2), water resistant, replace Space Gray with Jet Black, and of course the same display resolution as the 12.9" iPad Pro which will make multitasking more enjoyable.
As for hardware features, I'm hoping for more RAM on the smaller model, graphics capabilities approaching newer Tegra chips from Nvidia, even better Pencil capabilities, 3D Touch, a solid state home button (it really is better after using the iPhone 7 and weird when I push the mushy button on my iPad Air 2), water resistant, replace Space Gray with Jet Black, and of course the same display resolution as the 12.9" iPad Pro which will make multitasking more enjoyable.