. . . USA Indoor Champs then World Indoor Champs would be the logical answer.
Certainly the 3000. Maybe even a 1500/3000 double.
Since Grant felt he couldn't do justice this year racing both the Houston Half then indoors, certainly appears he has decided he couldn't do justice next year racing both World XC then World Indoors. So -- presumably -- he has chosen the latter.
One thing we all now know about Grants running: it is precisely periodized: fall base training, transition work in January, indoor racing in February & March, down time in March & April, base training again in April & May, transition work again in May & June, summer racing June into September, down time in October.
Grant tried to wedge GST into the April thru June time frame this year -- and he paid for it in September.
Strongly believe Grant will race USA XC -- then World XC -- sometime before his career is over. However, that might have to wait several more years until he moves to the roads and changes his periodization training to accommodate a different racing schedule.