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 learn about his brother Robert, who remained on Enroth, from Sephinroth. Then he will start looking 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 staying there and participating in the campaigns of the fifth game.