What is more, several things tip me off to look towards Gospels for the future.
1. Gospel of Judas. It looked as if study of the "other" Gospels from Nag Hammadi Codices had teetered out, but the publication of the Gospel of Judas by National Geographic (plus no small amount of sensationalism in the press) certainly reinvigorated the discussion. At SBL in 2007, I felt like I was the only person there who had not written a book on the Gospel of Judas. On the Gospel of Judas see the books by Simon Gathercole, April DeConick, and a good and sane intro is available from Peter M. Head, “The Gospel of Judas and the Qarara Codices: Some Preliminary Observations.” Tyndale Bulletin 58 (2007): 1-23.
2. Move beyond apologetic models. Discussion on the Gospel of Judas did open the subject of what is the difference between the canonical and non-canonical Gospels. It is a good opportunity to break down certain assumptions about the non-canonical Gospels. Not all the non-canonical Gospels were "gnostic" (most students I meet don't really know what gnosticism really is) and not all non-canonical Gospels are "heretical" (e.g. Gospel of Peter, Hebrew versions of Matthew, etc.).
3. Testing old assumptions. Nicholas Perrin's published dissertation on the Gospel of Thomas and Tatian, even if you don't agree with him (see discussion here), goes to show that there is still alot of work to be done in source criticism esp. if you take the time to learn the primary languages that are very rarely learned, i.e. Syriac, Coptic, and Ethiopic. This is better avenue for study than the tiresome hypotheses of a Q-Gos. Thom seedbed for Christian Origins prevalent amongst the Ivy League Gnostics. Likewise, Stephen Carlson's work on the Secret Gospel of Mark has shown how a bit of healthy scepticism can leave many assumptions of scholarship on shakey ground.
4. On publications, there have been a spate of introductions to the non-canonical Gospels and I recommend those by Hans-Joseph Klauck and Paul Foster. There has also been a number of publications of early Gospel Fragments edited by Thomas Kraus, Micahel Kruger, and Tobias Nicklas, Stanley and Wendy Porter, and by Andrew Bernhard which provide a good collection of the primary source texts for us to use.
5. Paul and the Gospels could be a big area too. David C. Sim continues to write a spate of articles articulating the anti-Paulinism of Matthew and I and Joel Willitts are editing a LNTS volume about the relationship of Paul to the canonical Gospels and Gospel of Thomas with an all star cast.
6. Christology of the Gospels. Synoptic Christology will be a new growth industry too. Simon Gathercole's book on Pre-Existence in the Synoptics and C. Kavin Rowe on Lord christology in Luke have begun breaking down the myths of the "low" christology of the Synoptics. The challenge is to situate the christology of the Synoptics in the context of early Christianity but also in relation to views of intermediary figures in second temple Judaism as well as in proximity to Graeco-Roman writings about divine figures. I myself would love to one day do something on the Marcan Jesus and the God of Israel. Though I expect Richard Bauckham will have much to say about that in his forthcoming two volume work on christology and monotheism.
7. Gospel Sources. I think that the field of studies in memory, orality, and texts provides many new exiting vistas for studying the Jesus tradition (see Tom Thatcher's study on John in this regard). Likewise, is it possible to adopt the Farrer-Goulder-Goodacre view on the Synoptic Problem (Luke used Matthew and no Q), but still retain a place for some shared written and oral traditions between Matthew and Luke?
8. The Gospel audiences is still very much uncharted territory. I think Richard Bauckham and co. have taken us beyond the "community" hypothesis in its 20th century form at the height of form and redactional critics, but there are still many hold outs. Bauckham was more careful on this than his critics suppose, he did not deny that the Gospels circulated initially among an immediate audience, only that they were not intended exclusively for that immediate audience. I've had my own dig at the so-called "Marcan Community". In addition, Edward Klink is editing a volume on the Gospel Audiences and it includes my own essay on the audiences of the non-canonical Gospels (Gospel of Thomas, Gospel of Peter, and Jewish Christian Gospels).
9. Many prominent scholars are moving into the field of Gospels research. Richard Hays is now working on echoes of Scripture in the Gospels, Francis Watson is about to start research on the non-canonical Gospels too, Luke Timothy Johnson keeps writing absolutely brilliant essays on the Gospels for various festschriften (see volumes in honour of Robert Morgan and Richard Hays), Simon Gathercole is writing a commentary on the Gospel of Thomas, Andrew Gregory will eventually publish some great work on the Jewish Christian Gospels.
So, if you're looking to do a Ph.D, my advice, start learning either Syriac, Coptic, Ethiopic and find something on the Gospels.