I don’t really see any positives beyond maybe making it more likely that there’s a capable main tank in the group. It would certainly discourage me from queuing either one.

The problems are many. There seem to be relatively few main tank players (and even from them, not all play main tanks exclusively), so queue times would probably get longer for everyone else. Choice of heroes would get reduced a lot (and there isn’t that much choice for tanks anyway). The definitions most people use for main/off-tank are different for high and low ranks of play, and pretty much whatever definition you use, some of them are on a grey area - and not always the same ones. It would also place more burden on at least the main tank - a problem that’s also present with the 1-3-2 composition that’s been talked about. The one potential positive certainly doesn’t outweigh the negatives, both potential and certain.