They have one, but they constantly change it when new people come into power because they think the lessons of the previous team don't apply to them. That is why Blizzard keeps repeating the same mistakes they've already told us about 10 years ago. Not all of it might be on the new guys though, some things may just me mandated, we don't know enough about their internal workflow to judge that, but it seems plausible enough.
Coming up with new things is a good idea as well. Just keeping the old stuff and doing it over and over gets stale extremely quickly. The problem simply arises from the fact that Blizzard styalized coming up with new stuff into a bad form of staleness, because every new system they make is not made with the intention of staying, it's made with the expressed intent of being thrown away after the expansion. So instead of innovating for progress, Blizzard has managed to make innovation itself stale.