I 100% believe that Blizzard is still more than capable of making a great game, so yes I believe they can make a good expansion. Whether they actually will use that capability has yet to be seen, though Shadowlands does seem to have some cool stuff that I'm interested in.
They make questionable design decisions sometimes, so we'll see, but I honestly believe they are capable of doing amazing stuff.
One thing Bellular said in a video that I think is very true (just citing where I heard it, think what you want about the guy and his channel). Blizzard is best when they stick to simple, straight forward, "cool," fun stories rather than a bunch of convoluted, complex, subtle and/or complicated stuff trying to tell a deep story. Hopefully Shadowlands is more in line with that idea than what they tried with BfA.