You mean Darth "I will force choke you to remind you of your place" Vader?
And the site of the final battle was the Imperial Archives, not the development site. That makes it more obscure and more removed from the "main storyline" action that runs through the SW franchise. Snoke is clearly more interested in getting a strong Force user than he is picking up the dude who could get swept aside by Tarin. And if Krennic did have Force connections, Vader(/Palpatine) would have taken interest in him, too (if only to eliminate him)...
Yea, Vader, whom Krennic felt he had enough influence with to go visit.
"The archives" was probably a cover story for the secret construction program. The true nature of the building was described when Tarkin called it out to targeting as the Citadel. Parts of Scarif were surrounded by a force field in similar fashion to the one surrounding the DS2 Endor site, which also featured a humongous dish tower. If Scarif was obscure before, it sure isn't now.
Snoke like Palpatine always pursued Force users, but this did not stop either empire from also developing planet killers. The main difference is that the First Order's technologies were derived from earlier Imperial stuff, some of which might have been provided by Krennic.
...Also, it would be thematically out of touch for any key character to make it from Rogue One to The Force Awakens (or beyond).
*Key* characters. There were plenty of nods to characters who would end up in A New Hope, but the main players in Rogue One died violently because the story really is about the boots on the ground war. EVERY member of Rogue One died to complete the mission. The Empire nuked the site as part of the cover-up, killing their own troops in the process. The cause survives. The people do not.
If you mean key Rebels, then sure, Rogue One was wiped out.
The key Imperials went on rule the galaxy in the original trilogy, with the exception of Galen Erso, who went out in a proper end-of scene. And Krennic supposedly, who was apparently disintegrated in the mother of all head shots. As if mega villains in the SW universe tend to get offed in charitably random incidents.