I think the reason why Araby and Kislev have not yet seen the light is because of Warmaster. Just like the naval battles could not be taken out because of problems with the copyright of Man O 'War, CA may have trouble obtaining Warmaster copyright, and that is why these two factions have not yet been able to be developed. At the beginning of total war warhammer 2, it is mentioned that Araby maybe will be a playable faction, they believed in being able to obtain warmaster rights, but this has been impossible for them. That is my theory.
It seems strange to me that Araby, being such a geographically important faction and having an entire army in warmaster has not been developed. The same with kislev, which was an obvious lesson for the first game, and their army is almost all in Warmaster. With Araby I do not believe all that nonsense that it is a political or religious reason, (especially since in Araby's most modern lore the arabyans are specified as godless and bereft of faith, and only the superstitious believe in the djinns). I think that if they have not developed these two factions it is because they have not been able to.