This paper investigates the challenges of designing mixed-presence environments for Mixed Reality and suggests future research directions derived from an expert workshop. Developing mixed-presence systems is a complex undertaking that combines the intricacies of both co-located and distributed mixed-reality spaces. Current literature in this field describes various promising design and development approaches but lacks a systematic overview, resulting in fragmented solutions to re-occurring challenges. Therefore, we conducted a comprehensive review of mixed-presence and multi-user remote mixed-reality systems, categorizing the prevalent challenges faced during the development of such systems, but also current trends, common use cases, study tasks and methodologies. Supported by these results, we then conducted an expert ideation workshop to collect and structure promising future research directions.As a result, we provide a detailed resource to orient and prepare developers for probable challenges and support researchers in making informed design decisions for future mixed-presence studies in Mixed Reality.