That's a fair opinion.
Hmm... a time period that might be interesting for sci-fi'd up lizardmen to make an appearance... *ponder pose activate*
I suppose it depends on how much of the classic Warhammer Fantasy setting you want. In theory, an interesting time period would probably be during the original war against Chaos, far enough back that you can do what you want. Except that then comes with the fact that it is far enough back in time that any of the recognisable aspects of the setting that aren't lizardmen (who'd be in their prime at that time) and High Elves wouldn't be a factor. No Empire, no Nehekhara.
During the time of Sigmar Heldenhammer? I wouldn't touch that period myself, because I prefer to keep some mystique of historical events that border on myth, but you aren't confined by the rules that I impose upon myself.
During the years before Nehekhara became the land of the dead? Explore an interesting part of history and get to annoy a pre-Tomb King Setra, who will no doubt declare that he doesn't kneel!
The Remas Empire.
Re-reading your initial post, the fact that you specify in Lustria specifically doesn't leave that much room for interesting periods of time. Lustria is fairly isolated from many of the big events and eras of the world. Pick the fall of a Temple-City, that becomes a big event for Lustria. The Vampire Coast's formation maybe.
I suppose another important question becomes "how much of an influence will they have on history?" Will you have their appearance re-write the history of the setting if they appear far enough back, or are you angling for a "forgotten history" angle?
Sorry, when presented with a loose idea, this is literally my thought process when thinking about possible ideas from a single prompt. My head is chaotic in the most interesting manner.
