I have the Sony BDP-S580 Bluray player and it already player mkv and avi files via DLNA, and I agree with the earlier mentioning of playing back media with the appropriate framerate (instead of playing everything with 60Hz).
As long as we can wish, how about iso support? I mean, the drive can already read the data from the drive which is basically an iso file. And the bluray drive already runs Linux (on armv6l according to the onboard Opera 9.80 browser) so a loop-mount is already possible with the kernel, making iso support probably easier to make that most other suggested features here. That is, if it is possible via DLNA.
I did have a problem playing back a blueray earlier, though. It had only DTS-HD sound and my amplifier at best supports DTS or DD (non-HD). Instead of downsampling to 2 channels (Dolby prologic), perhaps downsample to DD 5.1 or DTS 5.1?