The solution is to support increasing birth rates and provide state-led investment and other related measures to promote people returning to their periphery from the centre.
Migration is also a solution, but measures need to be taken to ensure the compatibility of the incoming population, an idea would be to focus on letting people in who are Christian Orthodoxes from former areas of the ERE (and elsewhere).
In the 90s, a lot of Pontics came to Thrace and Macedonia from the former Soviet Union and that was a very compatible population in the longterm, obviously nowhere near enough to reverse the demographic trends.