That rediscovery of the OT has sparked a healthy conversation about how the OT text casts historical (real) events and characters as emblematic of larger patterns in the development of the biblical storyline. (Hamilton's recent book is one place to see some of those issues under the microscope.) Trouble is, Covenant Theologians and Dispensationalists are inclined to polarized conclusions. Someone positioned between those systems might say that CT'ers flatten everything out and see too much continuity—types everywhere, descending into allegory. And D's often deny all types but those the NT explicitly identifies, disrupting the unity of the Bible.
One of the more thought-provoking discussions I've encountered is Jim Hamilton's 2008 lecture, The Typology of David’s Rise to Power: Messianic Patterns in the Book of Samuel [PDF] Audio's available here. Hamilton considers:
whether we are limited to the examples of typological interpretation seen in the Old and New Testaments, or whether, taking our cues from those examples, we can build upon them.In other words, does the NT identify every OT type, or should we look for the same kinds of correspondence that the NT writers identify, and apply them to OT figures that the NT writers don't? Or, should we apply the same hermeneutic to the OT that the NT writers did? If your answer is no, why not? And if your answer is yes, how do you know when you've crossed the line into unjustifiable allegory?
One of Hamilton's more important arguments against the limitation of OT typology to those specifically identified in the NT is that the OT itself interprets other OT texts typologically. On top of that:
[S]everal passages in the New Testament invite readers to conclude that the Old Testament is fulfilled in Jesus and the church in more ways than are explicitly quoted in the New Testament (cf. Luke 24:25–27; John 5:39–46; Acts 3:24; 17:2–3; Rom 15:4; 1 Cor 10:11; 2 Cor 1:20; Heb 8:5; 10:1; 1 Pet 1:10–12).Where the rubber meets the road on all of this is that Hamilton perceives several dozen points of correspondence between David and Jesus—right down to the minute details of the number of days between events. Were they all providentially ordained in history so that they might be recorded in the OT text as instruction both to its original and intended readers? I'm not fully convinced, and Hamilton concedes that he's not either. But he's convinced that some of them are, and that they're pretty important for how we interpret our Bibles:
It seems to me that typological interpretation is central to answering that question: precisely by assuring us of the unity of Scripture and the faithfulness of God—that as God has acted in the past, so he acts in the present, and so we can expect him to act in the future—we find the words of Paul true in our own lives:
"For whatever was written in former days was written for our instruction, that through endurance and through the encouragement of the Scriptures we might have hope. May the God of endurance and encouragement grant you to live in such harmony with one another, in accord with Christ Jesus, that together you may with one voice glorify the God and Father of our Lord Jesus Christ" (Rom 15:4–6).