Diablo 4 only needs an official announcement by now as even more intricate story details are starting to leak. For example, Lilith will make an appearance, following the one in Diablo 2.
Lilith will not have the same appearance as the did two Diablo iterations ago since this is apparently a reimagining of her form. Furthermore, it has had more than one creative mind during the design process.
This method is called plussing, as one designer gives the character initial traits and then hands it over to another. The second designer keeps the traits they liked, adds some ideas of their own and pass it on. According to one of the from Diablo 4 art book, Victor Lee is the original designer, with Brom adding his spin on the character.
Lilith technically appeared in Diablo 3 but only through her writings in Pandemonium Fortress during Act V. Therefore, she didn't appear in person and no character model was present, which is one of the reasons why the redesigned daughter of Mephisto should work without a hitch.
Furthermore, she is a shapeshifter that can appear as a monstrous creature or choose a human form that is accurate enough to fool just about anyone. Only the animals were put off by Lilith in her human form.
It remains to be seen what her role will be in Diablo 4 but more information should come during BlizzCon 2019. Let's hope there are no out of season April's Fools jokes in store for the fans this time around.