If you remember back with Sims 3, they actually did something very similar. Vampires were introduced with Night Life with the lore of Bridgeport surrounding the various vampires that shipped with the city, then introduced again in Supernatural alongside werewolves, witches and fairies. There was no functional difference between the two, though the two packs added different aesthetics that were available to any vampire if you had both installed.
Now that they're including a bunch of stuff into the basegame that only needs the EPs and GPs to unlock, they could move the basic alien code to basegame, have GTW unlock it since GTW aliens are honestly pretty bare-bones, then have the GP that focuses on aliens expand their powers, equipment, technology and add neat stuff for CAS and BB.
The only problem with this is that there's no precedent for owning a lot in a "secret" world that your stalker swarm can't access. Considering they can access a lot more than you can. When Noah Kane can run up to you and propose that you go to prom with him while you're in the middle of trying to unlock the third trap door in the Selvadoradan Temple (I'M AN ADULT, NOAH), or when three paparazzi are surrounding you with cameras while you're trying to talk to the First Order about a lightsaber, the idea of playing an extended game on Sixam with Geoffrey Landgraab doing a walkby once an hour just doesn't seem like it'll be very fun. At least I'm safe from them when I go to "secret" areas like Magic Realm, Sixam, Mt. Komorebi Peak, and the hermit's house in Granite Falls, but I can't actually live in those areas (technically I can with tents and all that, but I mean buy a lot and build a house without cheats). For the vision I'm seeing in this pack, we'd need a residential world that is only accessible as a secret area in order for it to work, lest your newly-generated alien wake up one morning to see Malcolm Landgraab and Wolfgang Munch trading insults on your lawn. Because unless steps are taken to stop this from the start, I guarantee you it'll be an issue.