My personal preference is always for a story to be able to stand as independently as possible from other works. I'm not against sequels, but there's something inherently weakening in my eyes when a film benefits so much from having watched previous episodes or connected stories first.

Unbreakable works as a standalone story and film; you could pretend its sequels don't even exist and it doesn't suffer for their absence. Split is nearly the same since its connections to Unbreakable are kept very subtle 99% of the time. Glass, however, is fairly dependent upon having watched both Unbreakable and Split first on a narrative level.

I still enjoyed it about as much as Split. There are portions (i.e. the second half) where Glass is so strong that it almost matches Unbreakable, but the first half drags a bit. Thematically, it's more complex than Split, but explores some of that movie's thoughts on the relationship and tension between skepticism and belief, with the supernatural existing as a mysterious zone between them. Shyamalan makes this part of the conversation easy for the audience to step into: a lot of Glass takes place in a mental hospital.

I think there's a lot to chew on thematically with Glass. Possibly even more than its predecessor films, even though I think Unbreakable is the better work overall. One interesting recurring idea is control of information and communication. How individuals and organizations shape themselves and others through the communication channels they understand and control. You can't control what you don't understand, so those who would exert control are best served by getting others to engage with them on a level they dominate.

For example, Dr. Ellie Staple excels in funneling others into particular forms of communication and aims to order herself and control others (mentally and spiritually, for the most part) by forcing them to engage with her on her preferred channels. In this way, she also controls information and thought. Elijah Price refuses to communicate with others for a large portion of the film, denying Staple any insight into his own information or thinking. He maintains control over his mind and spirit, even if his body is fragile and kept in physical containment by her. In her overconfidence, Staple fails to recognize that Elijah's non-communication with her is a strategic choice. Kevin (Et al.) and David are willing to humor Staple, but Elijah knows better than to give her any means of reading into him. He simply listens and reads into her instead.