A smaller footprint for the OS, resulting in some more space on our devices, combined with speed and performance optimisations would be my preferred choice.
Apple have hammered it for two years now with iOS 7 and 8. First the major redesign and then adding some seriously good features that effectively opened up the OS more. The time has come to take the last two years of hard work and make it sing.
I also feel that Apple should call time on A4 and A5 devices (Apple cutting A6 devices is doubtful). In fact I'd prefer to see iOS 9 be for A7 and A8 devices only, allowing them to focus on performance for the latest devices (64 bit) and leave the baggage of the iPhone 4, 4s, 5, 5c and iPad 2, 3, and 4 behind. Harsh, but we need to move on ... and lets face it, look around these forums and all you see are complaints of how slow iOS 8 is on iPad 2, 3, and 4 and older iPhone models.