The Lost Era was one of Marco Palmieri's projects. It's unknown whether there's still any editorial interest in continuing it. Indeed, we've already had a book set in that timeframe, Cast No Shadow, which didn't use the TLE banner. So while it's always possible there could be more fiction set between TUC and TNG, I wouldn't expect it to be called The Lost Era.
Anyway, we've already got fiction involving the ongoing border conflicts between the UFP and the Cardassians, notably Mosaic and Pathways by Jeri Taylor. And James Swallow's "The Slow Knife" in Seven Deadly Sins deals somewhat with Setlik III.