Horii has already said that DQXI is not a smartphone game, so there's really no point debating about that as if it's a possibility. But entertaining that line of thought, if there were eventually a mainline DQ game made exclusively for the mobile market, I think it would be very different in distribution and design than a typical DQ game. It would more likely be offered as a free prologue, where anyone can download the app and play the first couple of hours to get the hang of the game and the world/story. Then subsequent chapters would be distributed at a cost, along with microtransactions for cosmetic stuff. There would probably be seasonal quest events with leaderboards and prizes, rare drops at the end of dungeons, etc. The game could be designed entirely as a single player experience and still have many social hooks, like being able to loan out your character(s) to others, progress updates for friends, various activities which require a large number of players to complete the same tasks to overcome, etc.