I heard that the development team really enjoyed Red Dead Redemption, so i think we can expect to see some elements of RDR and other western games in FF13-2 before even FF15. they're even adding multiple dialogue conversations to FF13-2 (like Mass Effect) so i think if its well received they will keep using different inspirations from other games.
the thing is that the older FFs had a somewhat open world format with the world map system. so i think that they will reintroduce this feature like they've mentioned with FF13 Versus and FF13 Type 0. probably take some ideas from the way FF12 evolved this with a deeper system of exploration. while at the same time keeping the dungeons semi-linear like FF13-2 seems to be doing, or FF12 with some of its dungeons. which i think worked really well (Barhiem Passage, Rabanastre Waterway, etc).
i think now is the time they will revist the roots of the series and bring back things like chocobos, airships, card games, (hopefully better script writing), so in a nutshell i think FF15 will be a very nostalgic game, based on the merits of FF12 ( if Team 4 is back) and the merits of FNC and the other FF games.
