Never heard of them, but the exploration of the ruins of a long-dead alien civilization is something I would pretty much always love to see, even if I am partial to the post-human scenario. So these Iconians are much more interesting dead and gone than if they were discovered alive. Besides, with the rate of advancement in Star Trek, they would be near Q-like, and I am not very fond of Q-like species. Neither is likely to happen anyway.
Although... With the deep-space mission, setting a totally unrelated story in the ruins of their world might just work. Sounds even more suitable than Qo'noS for creating tension.
There's no particular reason they would have to be Q-like. Every species is a product of its own circumstances - there is no law that species always progress at the same rate, or even always progress at all. Humans in the 24th century are on an equal playing field with Vulcans, Klingons, etc, etc, despite arriving in space later. Bajorans have an ancient civilization which actually achieved space travel before the Cardassians - but didn't capitalize on that, and eventually wound up ground under Cardassian bootheels due to an incredibly large tech gap.
I'm not sure how canon it is, but there's a lot of assumptions out there about the Romulans having a relatively lower technology level than earth during the Romulan war - despite the fact that the Romulans had the technology to evacuate from Vulcan and found Romulus centuries in the past.