Interesting question. I'd like to see something a bit bigger, but I don't see them building anything much larger than a 900 based on the current architecture. Much larger than that and they'd need to add water cooling to keep emissions and noise down, especially if they want it to make any real power.
A 1000cc liquid cooled twin would allow them a lot of latitude though. They could build a Standard, an ADV and a tourer/cruiser using the same basic platform, much like Triumph does with their twin cylinder line.
BUT and here's the big one for me, right now they can get by with what's arguably one of the worst dealer networks in the industry, in large part because the bikes are easy to maintain and lend themselves to shade tree mechanic repairs and let's be honest the owners tend to be a bit quirky and take a perverse pride in owning a quirky bike.
If they were to build something more sophisticated, read that as expensive if you want, aimed at a more upscale market, they'd really need to improve the dealer experience, at least in the US.
FWIW Prior to buying my Interceptor I owned a Triumph Commander LT, it was a great bike, and on the surface should have been easy to maintain, but it was far from it. I've been a mechanic for over 50 years now, and that Triumph was the biggest pain in the ass to work on I've ever encountered. Just replacing the sparkplugs was an all-day affair and required draining the cooling system. If RE where to go down that path, it wouldn't be pretty and might end up being a sales disaster.