Who was the main person actually responsible for the idea of R1 and the story? Did Gareth Edwards have much creative input with regards to story or characters, or was he strictly there in a directorial capacity? In other words is there any real need to bring Gareth Edwards back in to aid in the Cassian series? People seem to love R1 and it's built up quite a following in the last 3 years, but it seems Gareth Edwards hardly ever gets a mention, unlike JJ and RJ.
As for taking characters from R1 and Solo there is definitely room there to use certain characters in stronger supporting roles than they were used in those films (e.g. Saw and his Rebels, Enfys Nest, Qi'ra, Mon Mothma, Bail Organa, Darth Maul etc.), but as others have said it would be strange to include characters like Jyn. For me the perfect cameo would be to re-introduce Ewan as Obi-Wan, as a teaser for his own series.