My Party is lvl 7-8 and I need ideas for creature that can shape change, is intelligent and has a high cr, since it's probably a higher level boss. It's pretty integrated in a city, has a house an collects stuff (artifacts, lesser creatures or other things/humanoids)

Any of my fellow DMs that has an idea? 

I have a couple ideas that come to mind. To get the CR right, how many PCs are there and do you expect the enemy to have minions/accomplices or be 1 BBEG vs party?

The Elder Oblex is probably a good one.

I had a similar-ish concept in my campaign. I eventually used a mildly tweaked husband-and-wife pair of Incubi/Succubi. They can shape-shift, and typically like to charm and manipulate others into doing their bidding so that they can amass wealth. This means there's scope for them swaying minions to aid them in battle.

For the campaign I was doing, they were embedded in high society, regularly throwing lavish parties to get more and more influential people under their control.

Ohh great idea, they are 5-6 players all experienced. Im also pretty used too, tweaking stats to either make it harder, easier depending. I'm also thinking there will be some minions involved. 

In that case, our recommended encounters are a little too easy, but you could use

• 1 Elder Oblex and 1-4 Oblex spawn
• 1 incubus, 1 succubus, and ~12 nobles or similar CR humans
• 1 Evoker with a shield guardian, you can use a different wizard if you like.
• A single Vampire spellcaster, but you can make them fight a Vampire or 2 Vampire spawn first.

Keep in mind, the balancing possibilities really open up if you have them fight the minions in waves first, then the BBEG in a separate encounter.

I use kobold fight club to balance encounters. It also gives you an idea of how many encounters you can expect your party to be able to handle per long rest.

How about a Mind Flayer Arcanist ( switching out one of the "standard" 4th spells with Polymorph, to get your shape change ability requirement ), since I assume they need the ability to pass amongst Humans.

By it'self that won't be enough for that party ( either 5 lvl 7 characters, or 6 lvl 8 ), but you'd want to pad that out with minions anyway. You could add 2x Mind Flayers as lieutenants. But Illithids are solitary "psychic commanders" which prefer others to do their dirty work, which fits in with your desire to create/use minion.

Are their minions likely to be humans, given the setting? Maybe a collection of Bandit Captains representing their enthralled ( and heavily experimentally augmented -but hey, what's the point in being an Illithid if you can't tinker with thralls ). They're only CR 2, so they might go down relatively fast if the Party is tactically clever and starts concentrating their effort, but if there are enough of them, the "action economy" can really tie down the Party, force them to demonstrate their abilities, and use up resources, while the cool calculating Illithid watches and applies their talents at opportune times and places, for the most devastating effect ( sounds like a psychic supervillain to me ).

2x Bandit Captains would be a Deadly Encounter for a party of 5 7th-lvl Characters, while you'd need 6 of them to make a Deadly Encounter for 6 8th-lvl characters.

Other monster which could stand in for human thralls ( with minor re-skinning ): Bandits, or Acolytes, or Thayan Apprentice.

As DxJxC suggested, you can play around with these in Kobold Fight Club to build a retinue for the Mind Flayer Arcanist which matches the character that seems right for the narrative, and keeps the encounter balanced.

If you can find an updated statline, a Steel Dragon could be great. As I recall, they loved to watch humanoids, and lived among them in plain sight. The more evil ones were highly manipulative. They kept their hoards in an urban lair, usually a manor with a room or rooms big enough to transform in. 

Alternatively, giving a lycanthrope a magic item and a few character levels, toss in a few minion lycanthropes, could be pretty good.

Hope this helps.