We know we're getting RS remakes by the end of Gen VI due to the DSi/3DS not being able to play GBA carts, but we don't know which generation they will be in.
My best guess to the release schedule at this point, going by Japanese release dates, is:
Summer/Fall 2013: RS remakes (DS)
Fall 2014: Gen VI paired versions (3DS)
Fall 2015: Gen VI third version or sequels (3DS)
Spring 2016: RG remakes (3DS) - 20th anniversary
Fall 2016: DP remakes (3DS) - 10th anniversary
Fall 2018: Gen VII paired versions ("4DS")
I doubt Game Freak has anything planned out longer than that.