The base game gets all kinds of content updates over the years, I would like to see that extended to expansion specific things too, that is better incentive to get more people to buy those packs than shoving advertisements in our face at every turn.
Like expansion specific objects that add gameplay elements not in the base game getting new options. Maybe some cross compatibility content too, new things found in one pack if you also own another one.
Magic realm for example could have an additional store that sells exclusively magical themed stuff from other expansions, it could also enable potions related to specific expansions, like to do with a specific occult. Magic realm could also seriously use some new magic themed clothing, though that could be a kit. Few clothing of that pack qualifies as something a mage would wear. Not even a single wizard hat.
Journey to Batuu has a lot of potential for additions too.
Kylo Ren's lightsaber being obtainable and usable for one. More lightsaber hilts and crystals would also be nice. Additional outfits could be added, like Rey getting an outfit she actually wears in the movies instead of some poor imitation.
More variations in missions too and some fixes to the ones that are prone to not work correctly, like characters you need not spawning at all or refusing to talk with you.
If you have both Journey to Batuu and University, you could be able to build a C-3PO looking protocol droid at the bot building table. Maybe there could even be a star wars themed bot building station available then to fit the theme.
Lightsabers should also get some uses in other packs, like being able to cut through the jungle with it in jungle adventure. Why would you need a machette if you have a lightsaber? Maybe when a vampire of werewolf tries to attack someone with a werewolf, that sim gets them to reconsider by igniting their lightsaber. Carrying one could offer protection from getting beat up, a fight only happening if both have a lightsaber or neither.