This seems exactly analogous to the CLISP/libreadline example I just linked in another comment, where it was agreed that CLISP needs to be under the GPL.
The situation is, probably, a bit different if the video player authors have no intention of using GPL code and just develop an interface, and someone else unrelated comes along and writes a GPL plugin. But if the app's authors ship the GPL plugin or worse write it, they're on shaky ground.
(This gets you into some slightly silly situations where author A writes GPL software A linking software B, software B can make use of a GPL-incompatible library C, all of A, B, and C can legally redistribute their software on their own, but Debian has to make sure A doesn't link C because they're shipping the whole thing.)
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This seems exactly analogous to the CLISP/libreadline example I just linked in another comment, where it was agreed that CLISP needs to be under the GPL.
The situation is, probably, a bit different if the video player authors have no intention of using GPL code and just develop an interface, and someone else unrelated comes along and writes a GPL plugin. But if the app's authors ship the GPL plugin or worse write it, they're on shaky ground.
(This gets you into some slightly silly situations where author A writes GPL software A linking software B, software B can make use of a GPL-incompatible library C, all of A, B, and C can legally redistribute their software on their own, but Debian has to make sure A doesn't link C because they're shipping the whole thing.)