An electronic gaming machine includes a game controller configured to (i) display, (a) an initial game play area including a plurality of reels and (b) a bonus region, the initial game play area including a plurality of active symbol display positions, and the bonus region including a plurality of inactive symbol display positions, (ii) generate a first game outcome for a first base round based on a first output, (iii) in response to determining that at least one of the displayed symbols for the first game outcome is a trigger symbol, unlock at least one of the plurality of inactive symbol display positions in the bonus region, and (iv) after a plurality of base rounds of the base game, initiate a bonus game including activating all inactive symbol display positions that have been unlocked, the activating including expanding the initial game play area to include the activated symbol display positions.