How is Dragon Quest 1-6 all connected?
There are certainly several connections through the Roto Series (Dragon Quest 1-3) and the Zenithian Series (Dragon Quest 4-6). The Dragon Quest 3 remix and 6 both have two worlds: an upper (dream) world, and a lower (dark) world. The lower worlds may either be literally underground, or they could be a different world. The dream worlds could both be Zenithia. There is a floating island in the remake of Dragon Quest 3 that could be Zenithia’s future or past. If the floating island is Zenithia then it would probably be the future of the upper (dream) world of Dragon Quest 6 (Zenithia) and the upper (dream) world of Dragon Quest 3 could be either unrelated to the Dragon Quest 6 game or the unlikely malformed lower (dark) world of Dragon Quest 6. Rubiss is in Dragon Quest 2 and Dragon Quest 6.