Minor issue that even talking about building ANYTHING in the area currently occupired by what we commonly refer to as har habayis, El Asqua, the Dome etc. would likely trigger a massive civil insurrection, war with neighboring arab countries etc. Other than that bloodletting (and also that there has been no sighting of Moishiach on Eastern Parkway other than the usual guys running around with yellow flags) initiating construciton of a “beis hamikdash” b’yamenu, would be a great idea.
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