I think sensible (aka not destructive to the quality of the main content/main campaign or to the quality/fairness of the in-game economy) candidates for DMC5 DLC are:
-additional characters, eventually
-additional missions (maybe one for a DMC3 retelling, one for a DMC1 retelling, one for a DMC4 retelling, or short mini-campaigns for new characters)
-retro DMC3/DMC1/DMC4 bosses and enemies
-retro DMC3/DMC1/DMC4/DmC weapons
-retro/bonus costumes
-decorations for the Devil May Cry shop, assuming it's somehow visitable/customizable in-game