Personally I think they should hold off the "xbox 1.5" until next year.
Do a slim revision for the holidays, wait for all the stuff to be sorted out with UWA and updates to the OS. (which if I'm not mistaken, is gonna be handled in two different updates, one this summer and another early 2017)
Introduce the Xbox10 as a new system and the start of a new gen with forward (for at least 2 or 3 years) and backwards compatibility with the Xbox One, and make it an actual complete upgrade rather than a half measure. The Xbone will have been out for almost 4 years, the length of time the OG was out before the 360 released. They get to keep the public perception of offering a traditional console with the benefits of having a lower priced option for budget users. I could see the Xbox One slim model dropping the One name completely.
This also would leave an interesting scenario in the overall console world, Nintendo would have just released its new gen system with NX, the PS4 Neo would have released, suggesting the PS5 is a bit of a ways off, and the Xbox10 could take the upperhand in terms of power due to this.
I just think trying to compete this holiday with NX and PS4 Neo both releasing is frankly just not a good idea in the long run. The Xbox brand has been hurt too much to realistically compete with either by introducing a simple upgrade to the existing one. They need a fresh start.
Id say that's why we aren't hearing much about whatever this upgrade could be, cause I just don't see it being at E3 with absolutely no information circulating about like how it is now.
All speculation at this point, but I'm glad things are to the point its fun to speculate like this.