It's Scottish football so ideal scenarios don't happen only path of least resistance outcomes based on pragmatism and selfish self-interest. The SFA don't believe in rigid boundaries so it all revolves around what the clubs and leagues actually want moving forward rather than on some masterplan imposed from on high.
The EoS don't appear to be keen on moving any further northwards so unless that changes Luncarty is as far north as the EoS is ever going to go. I'm skeptical whether the EoS would try very hard to hold onto Luncarty, Jeanfield, Kinnoull and maybe even Newburgh and St Andrews if they wanted to move into the Highland League catchment in much the same way the North Region were happy enough to let Montrose Roselea go, but no idea whether any of those clubs would actually conceivably want that.
My understanding is that at one point the HL didn't want the Tayside clubs but the Midlands League happened (timing was ideal in many ways given the North region was about to become a feeder) so nothing much is likely to change unless Brechin City get promoted to the SPFL but then relegated again and even then maybe they've changed their outlook on the whole Club 42 relegation issue after their time in the HL...