If, at the moment you say this to your roommate, you feel that you have a good idea and can confidently convince her/him that it's a good idea, use 'can'. If, on the other hand, you feel that you have to approach the matter more delicately because your roommate won't immediately agree, then use 'could'. In this kind of situation 'can' is confident.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.