I feel for everyone on this. I know you want it. I’d watch if it happened, but it won’t and here’s why:
This boils down to business relationships. Netflix already has a business relationship with The CW. So does CBS (co-owner). If Netflix thought for a hot second that a Grounder’s spinoff was doable, it would go through the hierarchy at WB/CW, first. If JR has legal rights (as their creator), he gets a say and he will get credit. There’s no avoiding it. If they did a spinoff, it would be entirely under their terms (their money). They’d deny even having seen the fan trailer. They’d keep far away from any fan ideas to avoid any legal concerns of credit.
Keep in mind, too, back in 2016, JR talked about writing an origin story (book) for the Grounders. This would be a complete conflict with a fan-centred project. He has the business relationship with the networks. His ideas would be the ones the spinoff is based upon (whether this book ever happens, who knows – his career is likely to see a slump after t100 is cancelled).
As far as ADC is concerned, this would be a backward move for her. The CW is unlikely to do any sort of spinoff (least of all something to please a fandom that thoroughly humiliated them), and the 100′s ratings suggest it faces cancellation, soon. ‘Lexa’ is a big taboo for that network now. The only way I can see ADC returning, albeit briefly, is if she is invited back for some flashback material or something in the finale, if warranted (like Clarke obtaining the flame). I’m sure lots of little boxes would need to be ticked off before she’d agree to it. Is this likely? No idea.
What might be likely is, a few years from now, if someone decides on a reboot. Someone with some vision who recognises what failed before and where it could be improved and decides to make ‘Clexa’ a proper part of the show. That could even be one of you. Even so, JR could still get some sort of credit.
I love this fandom, how rich and creative it can be, and I wish everyone on this project well and hope they find their way to applying their gifts to even greater things and, one day, taking over the industry.
Very best to you all.