I'm starting to think that the sentry himself is not the one you have to worry about. I think it is Bob. I think that Void and Sentry are just aspects of his mind, so there may be other forms in his head that we have not seen; kinda like the different types of hulks inside of Bruce Banner"s mind. We've seen the Sentry "Die" several times now but he comes back like nothing happened. This may imply that all that we have seen is just a figment of Bob's imagination materialized into reality; using this logic Bob might be more like a low level reality warper who can only change himself. I'm not saying he's not powerful, its just the opposite; Sentry and Void may only show a fraction of his true potential.
Sentry power theory
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