Civ 6 on launch utterly rolfstomps vainilla Civ 5, even with the initial hotfixes and patches, so I would say that it has a damn good chance of surpassing even my favourite, Civ 4, but let me elaborate on that.
I mean, yes, the game has some problems: Limited AI, "production is everything" balance (come on guys, 100 turn Pyramids, really?), some underwhelming Civs (Norway, Spain, etc), some underwhelming civics, really weak sea cities and religious spam / cluttering.
But all of these problems are a question of balance and adjustments, some of them very simple adjustments (like creating another unit layer for religious units), not design.
The core systems of Civ 6 are way better than the ones of Civ 5, and even some of the Civ 4. Housing is a better version of health, designing the AI to roleplay and being transparent is far better than Civ's 5 stupid, deliberate decision of making the AI opaque and designed in order to "play to win" and thus, has much more room to be improved, Civ 6 goverments and civics offer a great balance between the identity of civ 5 social policies VS Civ 4's goverment flexibility, and the new introduced systems such as districts and suzerain bonuses are the best part of the game.
So yes, I think that Civ 6 might very well surpass 4, and that it won't need even an expansion to do it so, just an extensive, Paradox-like balance patch (or fan made mod) will do the trick, you can quote me on this.