Very little in the way of Germanic writing exists prior to Germanic adoption of Roman literary systems -- there is Scandinavian runic, for example, but this does not exist in what you might call a literary form with developed prose. A 2011 article from the Indiana University Press is not quite on the topic of this discussion, but does demonstrate how hard it is to demonstrate the existence of any kind of runic literature. Thus, there may well have been Germanic thinkers, but there was little in the way of a local medium for their work to be propagated or to survive for us to know it existed.

However, there was interplay between Germanic tribes before the fall of Rome, so I suppose that is allowed for by the parameters of your question. The primary Germanic language that left behind a written record is Gothic, transcribed into Greek-style characters (as seen here.) The main example that comes to mind is more in line of what we would consider a religious scholar than a philosopher; Wulfila was a Gothic, or part-Gothic, Arian Christian who was ordained and sent to act as missionary and bishop to the Goths, translated the Bible into Gothic, and was thus a formative figure in the development of their Christian way of thinking.

Another figure I feel compelled to bring up doesn't fit the precise confines of your question. Boethius was from a Roman family shortly after the fall of Rome and the end of the Western Empire (born 480, just 4 years after the traditional end-date of 476 with Odoacer's conquest of Italy), but he served under the Ostrogothic king Theoderic with significant prestige until he was imprisoned and finally executed for suspected connivance with the Eastern Roman Empire. He is a prominent philosopher from his time, certainly no Plato or Aristotle, who wrote the Consolation of Philosophy. Again, this is a Roman writing Hellenistic/Roman philosophy, but I thought he was worth bringing up in case you wanted to investigate further.