In that case, I don't know. I've only played one FF port with an added optional dungeon that I can recall, FFIV. In that case, the extra dungeon was just an excuse for some new ultimate weapons. It wasn't particularly interesting and I suspect that optional dungeons added to later games might be similar.

What would be a good optional dungeon for the games? Well, for FFVII probably something extra in Midgar or Junon. Maybe to raid for some special Shinra weapons to help defeat Sephiroth (in other words an excuse for new ultimate weapons, I guess) or maybe a secret hidden dungeon in the City if Ancients. I always felt like there should have been something more there when you return in disc 2.

For, FFVIII, I think it would be interesting if they could work in an optional dungeon for Laguna somehow, but the logistics of writing an opportunity for that into the plot might be complicated. The moon idea also seems pretty good to me. Possibly also some additional time travel associated with the time kompression could be worked in. Like traveling to the time of Hyne or something (I'm not 100% on liking this idea,because I kind of also like the idea that Hyne may be a purely mythological explanation for sorceresses and not an actual figure in FF8's history).

For FFIX, I think maybe more exploration of Terra could be warranted and there could be an extra dungeon there or something.