Complex sentence: While construction was in progress, the Raptors and their partially completed arena were purchased by MLSE, which was contemplating building their own arena for the Maple Leafs to replace the ageing Maple Leaf Gardens.
Split into simpler sentences: While construction was in progress, the Raptors and their partially completed arena were purchased by MLSE. Prior to this development, the Maple Leafs had been contemplating building their own arena to replace the ageing Maple Leaf Gardens.