You know this is something I've been thinking about, but from a different angle.
Game designers are about to enter a hyper-fertile stage of development. What I mean by this is, Level Up, Black Flag, 5E all being compatible means that a third party designer doesn't have to stick to just one ruleset when creating their game. For example, I'm working on my upcoming game Crimson Catalyst (a D&D fork as well, but heavily stylized). In it, I don't need to just reference rule paradigms from core 5E. I can dip into Level Up (creative commons) for monsters, potentially draw on Black Flag's talents or other changes, and of course reach into other d20 games like PF2E or the OSR to find pre-made, roughly compatible material I can kickstart a design from.
Playing games with all these characters at a table would be interesting. I'm sure it will be more than possible depending on the operating philosophies of the table members. However, I think the real gold here is the absolute creative freedom designers and GMs now have to mechanically power their worlds.
