I've never understood where this "Apple hold back features" stuff comes from. In this case the OP's claiming they held back Retina displays... which is frankly ridiculous. The iPhone got a Retina display as soon as the silicon could drive it. The iPad got one as soon as the silicon could drive it (actually earlier, the SoC in an iPad 3 is possibly the most ridiculous chip design in the last ten years and Apple don't get anywhere near enough credit for that thing. The iPad Mini got it as soon as both SoC and screen would work in the confines of the smaller case. If you go back and look at the hardware specs available for each generation there just isn't a suitable option available to introduce the higher res screens earlier without significantly compromising the device.
By and large Apple tend to 'hold back' smaller things. Storage has been the prime candidate with iOS devices though there are a few others. Either way it's tough to see more than a couple of glaring examples (I'd say the only really obvious ones where 3G in the original iPhone and a camera in the first iPad but you could also argue Apple genuinely thought they wouldn't be needed).
I honestly don't think the Apple Watch will ever have cellular connectivity. It just doesn't make sense for most people as there are very few circumstances where you'd a) not have your phone, b) not have access to wifi and c) be willing to pay for another contract. Plus I'd imagine having to accommodate a SIM tray would be a major pain the proverbial. Slimmer, thinner, better battery and more powerful silicon is almost a given but those things develop naturally over time as the tech improves, no holding back there. Similarly GPS may come along but it'll be battery-dependent.
Other than that the most likely developments will be additional sensors (wearables make the most sense as ways to extend the ability of a control device to interact with the world), native apps (coming this year if I remember right, most likely with an intro at WWDC) and improvements to the manner and ways the Watch interacts with the iPhone.