It seems to me that Blizzard have a problem if they are intent on sticking with the mandate that they've earlier supposed (... that further expansions for Diablo III would bring additional content but not additional levels). The problem is inferno mode and the idea that an expansion would bring Acts 5+.
Is this a problem though? Well, yes and no. For some it's a matter of simply going to a previous difficulty to experience the new content; though that has it's own difficulties with the power curve whilst levelling being strewn all out of whack. However, what of those that are good enough to play in inferno mode but don't care for the long grind of gold and equipment to reach the final acts? For me, the temptation wasn't great enough. Act 4 is short and much less fun than the previous ones, so I have never pushed myself to complete it on inferno. (God, I'm so carebear) Won't players be expecting some form of instant gratification when they install their expansion but be disappointed if they find that they still have a long way to go before they can be permitted to enter the new areas?
What could Blizzard do to avoid such a scenario?
Perhaps they could make concurrent, rather than further consecutive acts. What I mean is, they could make an alternative Act I, which tells the story of the falling star in a different light. An Alternative Act II and so forth.
This could potentially be too much work for what Blizzard wants to sell to the customer but I can't help finding the idea somehow appeasing. However, the difficulty of tying the story threads together whilst making the new content suitably unique might be the major problem if such a system were adopted.
What form do you think an Expansion should take?