I thought they had considered a dive coaster, but changed their mind when Knott's got something essentially similar.
Really, if you ask me, they are in desperate need of more flat rides. Lots of other SF parks have been getting flat rides over the last few years (Endavour, Air Racer, Tourbilon, Wipeout, etc.). SFMM is way overdue for this, and it would be way cheaper than another coaster.