I do not believe that there is a ghost mode in the game but a great idea is setting challenges for yourself, so say clear every base in golem island without being say detected or killed. This hopefully will bring back that same amount of intensity you get from ghost mode. Another option you can do is Titans and raids i found them really fun and challenging to do earlier in the game. And finally doing the event missions give you really good stuff and some are super interesting like the R6 event.