When people speak of the centrum Paulinium (the centre of Paul's theology), they usually mean the centre of his soteriology: justificaiton by faith, union with Christ, reconciliation, etc. But perhaps the answer lies elsewhere. I'm struck by two "mothers" each claiming to be the maternal progenator of NT Theology. Martin Kahler said that "mission" was the mother of all theology, while Ernst Kasemann said that "apocalyptic" was the mother of all theology. Much like Solomon, how can one identify who the real mother of NT theology. In this case, much like a surrogate pregnancy, I think we have to have two mothers, esp. for Paul.
I think mission is the mother of Pauline theology insofar as Paul's call to go and be the apostle to the Gentiles is the central driving force of his theology. What is more, Paul's theology is done on the mission field (not in a seminary, college, university, or academy) and on the move and he has to literally walk and talk his way through several challenges (relating to pagans and Jews) and many crises (Antioch, Galatia, Corinth, Jerusalem).
Yet apocalypticism is the mother of Pauline theology to the extent that just about everything in the NT is pervaded by eschatology. Now, when Kasemann said "apocalyptic" he meant the shadow of the parousia casting itself upon the present time. But since Paul believed that Christians were the one's upon whom "the end of ages had come" (1 Cor. 10.11) then this eschatological perspective permeates everything. Now all apocalypticism is eschatological, but not all eschatology is apocalyptic. The apocalyptic aspect of his thinking comes through in Galatians, 1 Thessalonians, Romans (and I would even say Colossians). That is defined, chiefly, by a pessimistic view of the current age, a dualism of good/evil and now/then and heaven/earth etc. Paul believed that the God of Israel had radically acted in Jesus to save persons from the current evil age (Gal. 1.4; Rom. 1.17; Col. 1.12; 1 Thess. 1.10 etc.).
Are these the are the two "mothers" of Pauline theology?
I should also say that the centre of Paul's soteriology is a different matter, but with Marshall, Martin, and Stuhlmacher I'd probably say that "reconciliation" is the most elastic and comprehensive description of it. Although something participationistic based on being "in Christ" (and not to discount other metaphors too like sacrifice and justification) is also quite important.