Personally my money would be on the S4 using a Exynos 5 chipset.
Thing with keeping a phone "relevant" is that it's probably more perception than anything else. The S3 is more than half a year old and I'll bet it's still the best-selling "flagship" device, and in terms of its capabilities I can't see why it wouldn't be good for another 18 months. TBH I think we might be reaching the point where screens and processing power, at least on the high end devices, are good enough to meet anyone's needs for at least 2 years, which is why I like Moto's talk about factors like durability and battery life being priorities for this device. Plus "durable" implies that they don't see it as "disposable", which is good
The other thing that's needed for extended relevance is long-term software support. The Nexus is the poster child here, and I'd love it if they went that way. But for this purpose it doesn't have to be part of the Nexus programme, but IMO if they want this device to be what they claim they do need to make a public commitment that it will get long-term software support and make good on that.