You may consider using RFC4193 addresses inside your network.

There's also NPT but that's basically NAT which I, personally, recommend avoiding if at all possible.

I have already deployed ULA (and discovered a few wrinkles) and NPT is the only game in town for getting multiple WANs to work. You may recommend against it (I hate NAT too) but what is the alternative?

For example, you are a small business (say <£1M t/o pa) You have a 80/20MBs-1 FTTC (UK - VDSL, copper last mile) internet connection and you want to get some sort of redundancy/backup for your internet connection. So you get another line in at the same rate. In the UK, we pay around £30-£40 per month for this (if it is available.)

So, ISPa gives me a prefix and ISPb gives me another prefix. My router, via SLAAC, causes various addresses to become available but my endpoints are stupid - OK they do not know what to do because they are not routers. Knock out ISPa and they will not automatically fail over to ISPb. Nor will they do the really best thing and use both ISP links simultaneously.

I understand IPv6 very, very deeply (I've used it for years) and it is wanting. To be fair, the addressing scheme is spot on but the ISP addressing model is badly wrong and absolutely right at the same time (depending on who you are).

If you have a multiple GB connection and a /48 prefix through a 99.999% trusted supplier or a home grade VDSL link that you don't even know what is happening, then all is probably OK for IPv6. I look after rather a lot of people and businesses that don't come into those categories.

Is NpT really the best that IPv6 has to offer?