I don't think anything about the Foothills arena replacements has ever been announced. They're on the plan, but beyond that, I don't think anything related to the design has been decided.
Right now, they're part of Phase 3 of the plan, behind the Field House and the public realm rework, including revamping McMahon, so it's likely a decade or more before anything substantial happens with any arenas at Foothills.
The capacity of the arena there will likely depend on what happens elsewhere in the city in the meantime. Calgary would likely benefit from an arena in the 6-8,000 capacity range to take on the types of events that the Corral has hosted since the Saddledome opened, so it depends if something get built on the Stampede grounds, or elsewhere in the downtown area, in the meantime.
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Here's the plan for Foothills/McMahon that was presented to Council last September:
https://www.calgary.ca/content/dam/w...esentation.pdf
A return on investment analysis is supposed to be prepared for Q2 of 2022.