Sometime in the past week, the YYZ-KEF summer seasonal service has been updated to be operated by Rouge 737 for the month of June (except on Jun 3 and 5). It's looking like the next Rouge 737 base will be YYZ.

Other transfers from mainline to Rouge for May/June (likely not exhaustive) include:
YVR-SFO/LAX (1 of X daily)
YVR-YYC (4 of many daily)
YVR-YXY
YVR-YQB
YYZ-some Caribbean
YYZ-Mexico
YYZ-SAN
YYZ-DFW/IAH (1 of 2 daily)
YYZ-YOW (4-5 of many daily)

Schedules after June are still TBD, since the only routes showing Rouge 737 from July onwards are just YVR-Mexico.