How can we connect the fifth installment with the previous games in the series?

I was thinking about that too. Lord Lysander will search for his relatives and, through the Sephiroth, will learn about his brother Robert, who remained on Enroth. Then he will begin to search for a way back. In this endeavor, he will be joined by some well-known heroes (Sandro, Yog, Crag Hack, and others), and they will return to Enroth, with some of them remaining there and joining the campaigns of the fifth game.