For once the undocumented closed-source GPU drivers won't be the issue (or probably won't be) as this time around Amlogic licensed the right blobs from ARM and we already have them. The bigger issue is that S905X2/D2 hardware requires the Amlogic 4.9 BSP kernel, which in an evolution of all the low-quality Amlogic proprietary media code from the 3.14 kernel with additional new capabilities (none of which follow emerging kernel standards) and everything a bit more Android oriented than before. We can expect all the old bugs .. plus new ones. So the challenge is all about persuading good developers to take an interest in a(nother) bad codebase. You fix something, you break something else. You fix that thing, you break two things more. You fix those two things, something else doesn't work. So it's a shitty thankless slog, and with Kodi dropping amcodec support after K18 ships the large effort has a potentially short lifespan (far less than the run 3.14 has had). I'm not aware of anyone poking their nose into the Amlogic 4.9 kernel at the moment, so right now these boxes are Android only.
In the long-term there will be mainline support, but at the current near-glacial rate that Amlogic is submitting G12 platform code (which all requires nip/tuck rework, which incurs extra time) it's going to be "some time" before G12 hardware is mainline usable. There is also a conspicuous absence of media code being submitted, so we could be looking at a rerun of the current GX platform situation where Amlogic upstreams core platform support only and media support is left entirely to the goodwill of the community and a couple of vendor benefactors. It won't be quite such a drama as there's a moderate level of code inheritance from GX, but until core platform support gets further along we can't really start. I doubt we'll have mainline support for G12 boards within the next 12 months. Current GX boards will have moved to mainline long before then as the codebase is ~85% complete and so much easier to work with (and nearly all the major code components are written by LE team members). Once you've moved forwards and seen the light it's really (really) hard to resume an interest in the older kernels again.
NB: Apart from USB 3.0 support the main addition to S905X2/D2 boxes is hardware support for more codecs, e.g. HDR10, but support in hardware needs matching support in software and Linux doesn't support any of that stuff yet (and nobody has media to watch) so once you put the spec. sheet down and re-enter the real world there's no major compelling reason/advantage for S905X2/D2 over current S905X/D.
