As mentioned above there are tons of mounting options. When you use a gopro adapter, just go on amazon, aliexpress or banggood and you'll drown in options.
For dyno lamps: there are central mounting holters from Trelock and Busch & Müller, as well as special stems with integrated light holder from supernova lights (probably similar to the one from trek)
There are plenty of bar extenders, besides the amazon/chinastuff good ones are the topeak barxtender and minoura holders (e.g., the swing-grip).
A neat gopro mounting option is mady by ritchey ("universal stem mount")