If Darkness is weak to either Fighting or Grass by means of sub-types(types represented by a primary type in TCG), then I believe that if Dragons are to return, I believe Dragon should be weak to Water by means of the corresponding Ice sub-type that is Dragon's only weakness other than itself and Fairy-types in the video games. But even if they don't, change their new type's TCG weakness to TCG Water. At the same time, considering video game resistances, give Fire at least one type that resists them in the card game, even if that type has to be Dragons--or if they are removed from the cards, whatever is Dragons' SwSh TCG type..
^ Yes, I know it's Creatures' call, and not mine... However, I believe that the removal of Dragons virtually throws the game back to the RuSa TCG era, minus the no-supporter-nor-stadium-on-both-players'-first-turn rule that was made irrelevant when cards like Portrait-abilitied Smeargle(Undaunted) and Impersonate Sableye(Stormfront) emerged in a later Standard than that of RuSa's time... At least the second-going player can use supporters nowadays, but still, what a way to go backwards in a logical fashion. It's good to give card players a dose of old history, if only in parts, especially the -30 Resistance vs. the -20 Resistance, but at the same time, if you're reading this, Creatures, please consider the Water sub-type of Ice and make TCG's Dragons, whatever their type becomes in SwSh, weak to TCG's Water type and resistant to Fire, a type that Dragon is resistant to in VG, AND currently isn't resisted by any other type in the TCG as revealed so far…
^ I hope the rule makers have my two cents in their heart...