In response to a Square Enix member inquiring about whether or not Final Fantasy V and Final Fantasy VI were DS-bound, producer Shinji Hashimoto answered by way of Tweeting: “As for FF5 and 6, there are technical problems; presently these are also undecided.”
We wouldn’t take this to mean these games aren’t coming. The fact that “technical problems” exist indicates that they were, at some point, in production or pre-production. Although, whether “technical problems” refers to DS hardware limitations or is another way of saying, “We’re waiting for the next line of portables” is, of course, anyone’s guess.
Images sourced from the FF Shrine.