Thanks to all your suggestions, here are my thoughts now:
There are quite a few options, to help to narrow it down, I first ruled out the micro 4/3 systems (Olympus, Panasonic), based on the thinking that (correct me if I'm wrong) all else being equal, larger sensors produce better (lower noise) images at higher ISO. This maybe the old school thinking, I'm not sure if technology has advanced in recent years such that it no longer holds true - tell me if so.
Thanks to the suggestion by "PWPhotography", I took another look into Sony a7. While Sony 24-70mm f/4 ZA OSS Lens seems to be a great lens, Sony doesn't offer anything wider than 24mm for FF, that's why I ruled it out before. With Voigtlander 15mm, this combination becomes viable again.
I also took another look at Canon SL1. There is certainly advantage to go with Canon, as any other system would require a learning curve and change of work flow etc. While the body is light, battery life will be better than the Mirrorless cameras, Canon doesn't seem to have great lens for the UWA range, compared to the other vendors. Going with the 11-22mm lens (10+ yr old technology) seems to sacrifice too much in IQ, and it's not that wide on APS-C.
Fuji seems to have a great reputation of its lens quality, with the 10-24mm zoom, I could get the 15mm UWA I really want. However, its 18-55mm is not wide enough for me. As I've used the 24-105mm on FF most of the time, 24mm is really a sweet spot for me, as it's wide enough for 90% of my wide shots. Fuji's 16-55mm f2.8 is the right range for me, but weights twice as much! the 16-50mm seems to be much less quality in IQ. The other dilemma is, I like the 24MP sensor of X-PRO2, but want the articulated LCD and the 35mm camera feel of X-T1. The folks on Fuji forum suggest me to wait for the next model X-T2, but I'd like to get a new system for my trip next month...
So it's a choice between the Fuji X-T1/T2, and Sony a7.