Expensive, but one of the best ways to do mesh (I haven't seen it done, but it MIGHT be) is to you multiple radios. I don't simply mean cover in 2.4GHz and backhaul on 5GHz or something, I mean multiple radios.
So you can, say, cover on 2.4GHz channel 1, 20MHz, but then have dual backhaul on 5GHz, a radio on channel 36 and one on channel 141 for example. So that way you can both send packets down the line on one channel/radio and accept packets from further up the mesh on the other channel/radio. That way, other than latency, you have no penalty no matter how large you make the mesh.
I've seen DIY solutions like this for wifi long link where you don't have line of sight. Setup bridge in the middle connecting upstream, wired to a bridge going downstream and having them operate on seperate channels so they don't interfer at all.
It would be interesting to see this all in one product (hey, look, XStream already has one 2.4GHz radio and two 5GHz radios...)
Gets pricey, but it would be "ideal".
For maximum expense and wow, have more than just 3 radios, so you can backhaul using two on seperate channels and then extend both 2.4GHz and 5GHz on other channels as well. Sucks spectrum, but it would be cool.