For what it's worth, I think the best way to do a new FRCS is not to have a FRCS at all.
Instead, split the FRCS into the following (massive, 300+ page) books:
1. Atlas of Faerûn - detailing the geography (physical and political) of the continent, with maps showing different eras (Crown Wars, Netheril/Jhaamdath, Post-Netheril, Post-Spellplague, Post-Sundering) and 1- or 2-page articles on each significant country or realm that arose during each era, as well as 2-page articles on major cities (Forgotten Realms Adventures, the 1e to 2e transition book, uses this format - and I still use it to this day when I plan adventures in a certain city). No crunch whatsoever, make it edition-neutral.
2. Faiths of Faerûn - a volume combining the 2e faiths trilogy, detailing all gods (living and dead) and their churches throughout history. Bonus points if deities that have never appeared in print (at least not in detail) such as Murdane and Auppenser are shown. No crunch or very light crunch (e.g. new domains).
3. Heroes and Villains of Faerûn - this one is for the novel fans, 1/2- page to 1-page biographies of every major NPC ever to have appeared in the Realms, from all the different eras. Crunch in the form of statblocks.
4. Secret Societies of Faerûn - combining 2e Cloak & Dagger and the 3e books (Lords of Darkness, Champions of Ruin/Valor), this book provides info on every secret society ever to have been active in the Realms and their history. Crunch in the form of statblocks and magic items.
5. Magic of Faerûn - all the different magic types, changes to magic throughout the ages, famous guilds and bard colleges, spells, magic items, new schools of magic, warlock pacts, bloodlines, and so on. Fairly crunchy.
6. Swords of Faerûn - significant wars, warrior schools/martial traditions, mercenary companies, equipment, new fighting styles, other martial stuff. Fairly crunchy.
One release every 3 months or so, then continue with regional books, starting with regions that haven't had a regional book in forever such as, oh I dunno, CORMYR?