I think we need type definition for app to constraint its API like use, req and res or at least doc/comment to describe it, so that user won't make any mistake to use a customized server middleware.
And actually connect is a very pure middleware, it won't have much influence in performance except that it doesn't support async handler (which can be trickily implemented by calling nextafter await/in promise).
As customized server middleware may not be as efficiency as connect, maybe we should consider if it's necessary to open this door as it's similar to programmatical Nuxt usage.
If the customized server middleware is better than connect, we can just replace it with new one since app is very common (a server layer) and won't include special logic.
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