My personal favourite would be Codex: Adeptus Mechanicus but I would love to see the codicies overhauled to:
Codex: Defenders of the Imperium (Guard, PDF, Arbites, Ecclesiarchy)
Codex: Servants of the Imperium (Inquisition, Grey Knights, SoB, Deathwatch)
Codex: Adeptus Astartes (Codex oriented chapters: Imperial Fists, Ultramarines, etc)
Codex: Angels of Death (Non-Codex Chapters: Blood Angels, Space Wolves, Black Templars etc)
Codex: Rogue Traders (Squats, Mercenaries, Rogue Traders, etc)
Codex: Mechanicus (Adeptus and Dark - all you'd need is a few options for daemonic possession etc)
Codex: Lost and the Damned (Chaos cults, Blood-Pact esque forces, mutant hordes, corrupted preachers, Daemons etc)
Codex: Chaos Legions (Emperors' Children, Iron Warriors, World Eaters, Black Legion, Thousand Sons, Night Lords, Alpha Legion, Death Guard, Word Bearers - mostly 30K tech, smattering of more modern stuff)
Codex: Chaos Marines (Renegade warbands - tech identical to loyalists)
Codex: Eldar (Craftworld, Exodite, Harlequins)
Codex: Corsairs (Dark Eldar, Pirates, Corsairs)
Codex: Tau Empire (lots more on the Auxiliaries)
Codex: Tyranids (with Genestealer Cult list)
That would be 15 codexes covering scores of armies, split into three main groups - Imperial, Chaos and Xenos. It wouldn't be difficult to also include rules in the main rulebook for using allied forces, either.
In terms of new models, well, some would require more work than others (Mechanicus, for example). But overall it probably wouldn't be worse than the DEldar reboot.