An Existential threat?
No, certainly not. Iran and Saudi are rivals, but Iran has neither the means nor the inclination to be an "Existential Threat" Likewise, I do not believe they are a threat to Bahrain because Bahrain has both British and American cover, and we're more likely to help them than the Saudis in a pinch - for strategic and historical reasons.
Iran might try to grab parts of Iraq but only if Iraq actually disintegrates and in fact it doesn't look like it will.
Suspciaion of Iran seems to me to be rooted in the fact that their are neither Arab nor Sunni, but then this comes back to your definition of "Arab" where in fact it could be argued that a Shia-governed Iraq is naturally more likely to align with Iran in any case, both being neither Sunni nor Arab.
Bookmarks