Agree Canon could serve itself well to take mirror-less more seriously.
I don't think a Full Frame Rebel DSLR would compete in the Mirrorless market at all.
An FF Rebel would need to be around $1K or less to sell. $2K is the 6D market, which already exists.
Not sure why "full frame" is a required part of the equation.
Fuji and Olympus don't seem to be hindered in this regard.
SL1 was sadly pretty much a failure. Selling point for that body has dropped to around $300.00 these days.
That said, an SL1 with a pancake is very piortable and offered decent IQ.
Problem with SL1 and any "rebel" solution IMHO is controls. I really don't like shooting my SL1 due to it's need to use menus to make most adjustments.
Take a page from the Fuji camp, the Xt1 etc. where all controls in this smaller than an SL1 body are easily located and adjusted. With a body like the Xt1, I simply can't take the plasticky menu driven SL1 seriously as any sort of Mirror-less competition.