Stadiums in GTA have mostly been visible but not very interactive. The Vice City Beach Stadium in the 2002 game had derby matches. Maze Bank Arena in GTA V hosted side events. We see what appears to be a stadium frame in Trailer 2. We read things carefully and don't make outlandish claims.

For sports venues throughout the map, see the stadium locations page.

What the frame shows

A large circular or oval roofed building. Big enough to read as a major sports facility. You can see parking infrastructure around it. Limited frame time - no clean exterior or interior shots.

A plausible real-world analogue would be Hard Rock Stadium or Loan Depot Park, both actual Miami venues. The frame in the trailer is too short to pin it to a specific real-world reference.

What stadiums historically do in GTA

Venues for concerts. Broadcasts of sports events (sometimes interactive, sometimes not). Arenas hosting side content - derbies back in 2002, races and fight events in V. Occasionally story missions use them as a backdrop.

Stadiums have been used in GTA Online for Diamond Casino events, Arena Wars and the Stunt Race series. None of that confirms anything for GTA 6 - it's precedent.