Since looking at some mods and the assets for dragon knights in Original Sin 2 I was thinking a dragon companion in the DnD universe would be very interesting. Since dragons can shapeshift into human forms it could be implemented as some polymorph equivalent for gameplay purposes. But I think such a companion would have the most interesting companion quest as the dichotomy between chromatic/evil and metallic/good dragons is part of the DnD lore that has pretty much never been explored or explained. Having a typically evil dragon join the party in an attempt to overcome their evil nature or having a good dragon that is more selfish and resentful of other races (or both) would be very interesting, probably one of the most interesting party members I could envision in BG3. The only problem is such a companion, if Larian went through with it, might be TOO popular. Then again... is that necessarily a bad thing?
