Regiments of Renown are a mechanic in the game. If you get your lord up to certain levels they unlock RoR to use depending on the level. They are usually tailored to the army you are playing though (so Empire RoR choices are suped up Empire units with unique abilities and maxed out experience).
https://totalwarwarhammer.gamepedia.com/Regiments_of_RenownAlso after the Empire Faction rework, if you conquer/confederate different Elector states and put in an Elector Count, you can hire region specific units like The Knights of Moor if you control Sylvania and Carroburg Greatswords if you own Reikland, along with buffs and a special runefang for each province.
I could see them do a DoW army that acts like a horde faction but I wouldn't count on it. They usually add factions based around a Legendary Lord to lead them (So Karl Franz, Volkmar the Grim, Balthasar Gelt, and Markus Wulfhart for Empire, Louen Leoncour, Faye Enchanctess, Repanse, and Alberic for Bretonnia, etc) with special unique abilities/strategies and the Empire is probably going to get new ones based around the Storm of Chaos/End Times like Valten, Boris Todbringer and Luthor Huss. Markus Wulfhart is close to a DoW army I suppose considering he's in Lustria as an Empire like conquistador army.
There is a good deal of speculation that Kislev will get to be a playable faction in 3. And with GW revisiting WHFB and potentially making a legit army for them, I could see CA making that possible.
I agree that having Araby in the game would be awesome but I've heard that CA is worried about coming off as racist for doing it. Here's an article where CA said that currently there's no plan for it.
https://www.pcinvasion.com/creative-assembly-confirms-araby-wont-be-coming-to-total-war-warhammer/I also think Amazons are a pipe dream and is more likely going to be the work of mods. Same with DoW horde army. But then again the same was said about Norsca and Vampire Coast armies and yet they're in the game as fully playable factions with their own lords, units, and game mechanics. So who knows! They may get added later.