UA used to enable a DEN stopover with 10k mileage plus miles for domestic itineraries. I wonder if it's coming back or if the idea will even get expanded to other options.
I don't know if it's a rule, but in my observations, stopover options have always been at hubs. All of my stopover experiences have been overseas flights like an SFO stopover going MKE-ORD-SFO-SYD on UA. The Asian airlines seem to make stopovers easy to arrange with both FF 1-way awards and revenue flights. I've done it with CX, CI, QR and SQ. The European airlines...I've tried many times and have succeeded only once with a stopover at AMS with KL going from LHR-AMS-JRO, with CO award travel...that was a few years ago.