Anyone imagining it'll be free-roaming like XI or even an outright clone is probably in for some disappointment.
Looking to other mobile games, I'd expect each zone to be broken down into multiple key points. At said points, you could do something like fight mobs for EXP, tackle quest objectives, or even do things like harvest materials. Unlocking other nodes would probably require winning a fight or completing some objective, but you can safely bet there will be some kind of stamina feature tied to it, too.
Actual fights are probably going to be more like the solo player teaming up with various storyline NPCs with the ability to "borrow" a friend's character to fill a party slot. Fighting itself will probably be similar to other ATB entries, if not outright turn-based. This'll also mean no stuff like linking adds or getting magic aggro.
In the end, XI's stories can be told in this format, which is likely more important than trying to relive the good ol' days of MPK or AH manipulation. Actual player trading will probably minimal for fear of RMT, but who knows. Basically, think of an XI you can sit down and play for 10-15 minutes at a time. Anything greater isn't particular conducive to mobile play, especially with battery drain and variable internet access.
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Violence good. Sexy bad. Yay America.