What replicant hardware support do I miss the most ?
First : wifi, then GPS.
it would be nice also to do videos and not only photos
Here are some details about the status and possibilities for each component.
Wi-Fi can indeed be solved by using an external ath9k_htc adapter. Some more userspace adaptation would be needed to make it easier to use for end-users.
GPS could similarly be solved by using an external dongle and connecting with gpsd. This also requires some userspace adaptation work but it's quite doable.
Regarding video capture, it's unlikely that it will happen soon as the CPU is simply not powerful enough to encode the video while receiving it. However, we could allow the media coprocessor to work when its firmware is in place (currently, it's explicitly disabled).
Regarding video playback, some work could be done there. Also, the VLC application often does a nice job.
Faster graphics is also an area of work. llvmpipe is showing some signs of improvements. Wolfgang, who's working on Replicant 6, knows more about that topic.
For better modem isolation, there are very few choices except using devices that were designed witch such isolation, such as the GTA04, Optimus Black or Neo900. Those are not well supported at this point by lack of time.