updated 09:31 am EDT, Wed July 30, 2014
Potential specifications unknown
The same Apple support document that references a 2014 Mac mini also lists a "mid 2014" 27-inch iMac, checks show. It's mentioned alongside a 2014 21.5-inch iMac, but that presumably refers to the low-end model Apple introduced in June. To meet a mid-2014 launch date, Apple would have to ship an updated 27-inch system within the next month.
What kind of specifications the product would have are uncertain. Apple could theoretically create a cheaper 27-inch model, but the company tends to reserve that size for its upper tiers. More likely may be slight specification boosts, in the same vein as the new MacBook Pros.