What are the issues for Paul? To oversimplify, but I hope helpfully: Much traditional treatment of Romans and other letters assumes that Paul is mainly concerned with individual soteriology, while recent Pauline scholarship emphasizes that Paul is concerned about the plight of Israel and the purposes of God among the Gentiles. How to settle a global interpretive question like this? The problem is, both approaches can offer more or less coherent accounts of the letters themselves. So long as the focus is narrowly on the letters themselves, these kinds of debates seem hopeless of resolution. (Perhaps a Kuhnian “paradigm shift” would be applicable; in one paradigm anomalies accumulate until a new paradigm takes its place that explains the anomalies.)
The best way to address this, I think, is to see that Paul was writing out of the conviction that the gospel fulfills OT hope and promise. Whatever Paul means by “gospel” must be the fulfillment of what Isaiah, say, prophesied about. So, the question is, What did Isaiah prophecy about? He prophesied about the demonstration of God’s righteousness among the nations through the restoration of Israel, about the restoration of Israel through the suffering of the suffering servant, about return from exile, about a new heavens and a new earth. Paul’s gospel is that all these things are taking place in Jesus.