WTF? So Sarek was so pissed off about Spock deciding to join Starfleet that father and son didn’t speak for decades... and then Sarek himself ends up serving as an officer aboard the Enterprise? How the heck were they planning to explain that? Or was this to be yet another new role for Lenard?
It hadn't been decided. But... to save money on ear molds, actors who had played Romulans/Vulcans (eg. Lawrence Montaigne as Decius and Stonn) were often invited back, and Mark Lenard had proven very easy to work with in his two TOS appearances. The producers were also impressed by Lenard's resemblance to Nimoy.
Whether Spock's position on the ship as resident Vulcan was to be filled in Season Three by Ambassador Sarek, a recast Spock, or a brand new Vulcan character, had not yet been decided, but it was all part of a threat to Nimoy that he wasn't as indispensible as his agents were trying to suggest.