Glute medius is undoubtedly an important muscle; being the stabiliser of the hip and pelvis in the frontal plane. Weakness of glute medius is functionally very detrimental and metabolically costly - excessively movement in the frontal plane is never ideal.
So how do we go about improving the strength of glute medius? Firstly, we have to realise that although glute medius is a stabiliser more than anything, it still has a very large cross sectional area. A large cross sectional area means there's plenty of room for developing some hypertrophy within the muscle. A muscle that increases it's cross sectional area is more capable of both force production and shock absorption; the latter being more relevant to glute medius.
Progressive overload does not always have to be adding weight to the bar. Exercises can be used to manipulate the difficulty. If an exercise creates fatigue within the 8 to 12 rep range, this could be considered effective for developing hypertrophy.
Below are some suggestions of how to progressively overload glute medius by increasing the difficulty of the exercises. Notice how the progressions involve moving further into positions of inner range hip abduction. Why might this be effective? Based on the length-tension curve, a muscle working in inner range is less capable of producing force. However, if one attempts to produce force in this range, more active muscle fibres will need to be recruited to produce the force than a muscle working in a more favourable length tension position. Therefore, more active muscle fibres could suggest more chances of muscle hypertrophy - the initial goal of creating a specific adaption within the muscle.