I like Fuller. His skillset is like a more gifted version of Jermaine Kearse. Six feet tall, uses routes to get open, high points the ball perfectly, and is very easy to trust on the deep ball.
That said, I'm still a big believer Richardson fitting Seattle's new offense like a glove, and if he can stay healthy it would give Seattle a very good WR trio. During Wilson's recent hot streak, he's spread the ball less than before and almost all of his yardage and TDs have gone to his top 3 WRs, with the #4 and #5 WRs getting almost nothing.
Even if Kearse doesn't come back, the WR group could be pretty crowded next year. I like Fuller a lot but it would be a luxury pick, IMO.
I haven't researched this draft at all but my top two areas of need for Seattle right now are CB and pass rusher. It's very hard to find a good pass rusher in the late 1st though, so Seattle should probably try to address pass rush in trade or FA.