A Fire Emblem fan has recently been looking at the internal codenames for previous entries in the series and has discovered that there is potentially another Fire Emblem game in development at Intelligent Systems that was being worked on after Fire Emblem Three Houses development was wrapped up. Fire Emblem Three Houses was known internally at Intelligent Systems as Iron17 and Fire Emblem Engage was known internally as Iron19, so that just leaves Iron18. It is thought that Iron18 could well be Fire Emblem Genealogy, which has been rumoured as being in development at Intelligent Systems for quite some time.
Fire Emblem codenames suggests another game was in the works after Three Houses

Do we need know what Three Hopes’ codename was?
iron 17 and a half
Not sure, but because it wasn’t a mainline Fire Emblem game and therefore a spin-off, it apparently didn’t have the internal codename of Iron18.
While I am all for a remake of Fire Emblem Genealogy of the Holy War, I really hope Roy’s game gets a remake at some point.
Same here. Maybe the remake can make it have nods to the prequel with Roy’s dad. Maybe add an older Lyn to the remake, too.
If I had to guess, I would say “Iron 18” is a remake: now whether or not it is the Holy War games or not… I’m not touching that debate with a 10 foot pole