Riku Sanjo is knocking it out of the park thus far with W, but he's not a machine. He's produced some stuff that is good and some stuff that is quite bad at different points in his career, like most creators.
In particular: one of Sanjo's writing credits is M.D. Geist, long considered by many critics to be one of the
worst anime ever made, or at least one of the worst released in the US. He also claims
co-creator status for it and wrote the sequel.
You can't excuse M.D. Geist as an early credit, because he'd already done the great Dai no Daibouken series by that point. Riku Sanjo knew how to write, and especially how to write good action. He just somehow woke up one day and thought that writing M.D. Geist would be a good idea.
So it strikes me as pretty far from impossible that Accel will turn out to be a bad idea. I hope he will be a good idea, of course, and I really hope his character was designed when all of the other core cast were. That would increase the chance that he has something to add to the plot.
Pumping in new stuff to sell toys is fine; a Rider show can plan around that and create good drama from it. It's when you pump in new stuff
and attached new characters that your creative team can end up stuck performing a delicate balancing act, if the additions weren't planned out well.