This paper explores European Union (EU) enlargement to the Western Balkans and its potential implications for EU decision making. We first develop game-theoretical, spatial models and examine how the future enlargement may challenge the institutional structure of the EU. Second, we analyze the effectiveness of recent proposals that attempt to mitigate the inefficiencies in decision-making that may result from enlargement, such as the "Sailing on the High Seas" proposal submitted by a Franco-German Working Group on Institutional Reform. Subsequently, we use Chapel Hill Expert Survey data to apply our models to various Council configurations. Furthermore, we consider the impact of the enlargement on the budgetary net-contributions of member states and how potential changes in the net-contributions will in turn affect member state voting. Our analyses address important gaps in the literature, the lack of formal and quantitative studies of the impact of the enlargement on the efficiency of EU decision-making. The findings reveal that enlargement to the Western Balkans is likely to put pressure on the Council’s legislative efficiency, and that the proposed institutional reforms may offer only partial solutions. Specifically, we conclude that the "Sailing on the High Seas" proposal may have a limited effect in mitigating the structural challenges created by an enlarged EU. We conclude by proposing a number of more effective institutional changes to preserve the EU's ability to govern in a larger, more diverse political landscape.