Bloody Palace was already post-launch DLC for DmC, albeit free. They got people to preorder and pay for Trish's costume which was already within the game files, which to me suggests they were experimenting with what they can get away with.
If Capcom is PR savvy they'll make anything that makes use of pre-existing assets free, while everything that's new content (costumes not found in the base game, weapons, new levels, bosses, etc.) is premium (season pass or single piece DLC).
Personally I think we should start raising a stink already now. I believe in pre-emptive measures. Tweet Itsuno, Kobayashi and the Capcom and DMC accounts, threads on Capcom-Unity, make a topic on GAF about it (we know Capcom reps browse the forum) regardless of whether we have official word about DMC5 or not.