In Mass Effect 2 and 3 the Citadel often provides a break from gathering your troops and war assets. It has shops and offices and its main function was to provide a place for conversation to progress the story and as somewhere to meet up with other characters rather than to give us side missions. In Mass Effect 4, it would be a great idea to introduce a larger Citadel with more opportunity for exploration and side missions. It should update organically with each visit and fit in with your place in the games main storyline. Alongside this, BioWare could reintroduce Omega and Illium as hub planets with side missions available, exclusive to them each adapting to your place in the games narrative. For example, posters around the planet detailing political could change and the conversations you hear when walking by residents should mention events that have happened in-game. This is by no means a revolutionary change but it is certainly an area in which Mass Effect has been lacking in the past.