This is the key, I think.
For me, Rogue One was interesting in that it proved that you can take existing characters from other Extended Universe stuff and use them in central roles, as long as you introduce them effectively for the audience of the movie too. Saw Gerrera was first introduced in the Clone Wars cartoon in 2012, but I didn't need to know that (or anything about him) to understand and enjoy his part in Rogue One.
Equally, I know nothing about Ahsoka Tano, but even if she was put in more of a lead role than Saw Gerrera was, it's not a problem. I knew nothing about Jyn Erso either, before I watched the movie. A movie audience doesn't care if the character has been drawn from a (relatively) obscure extended-universe series, they just care about how the movie works on its own terms.