I think that by the end of the year, if Sanders hasn't made any real progress by then, he'll endorse Warren.
The question will be who will her running mate be then?
If Biden isn't interested, and he hasn't shown any signs that he's all that interested in running for pres these days, the only candidates that bring anything to the table is Harris and Gabbard. (Yeah, it's all about the women here. The guys aren't impressive to me.)