I cannot find explicit discussion or a timeline on the future of an internal Blender muscle rigging system, other than it may be planned for 2.8x.
In which case, I feel it’s important to discuss what features a muscle rigging system should have.
Personally, I’ve had need for the following:
• Curve Controller — Simply a curveable ‘bone’ that stretches OR extrapolates the curve between it’s two control points (head/tail). When the length shrinks or grows relative to rest length, the curve should either buckle like the current IK constraint does, or extrapolate/interpolate the curve (not scale). The curve is set via control point(s) in either bezier or nurbs fashion or with fall-off methods. Ideally, these control points can be hooked/parented to anything, and have their position(s) defined as a % along the muscle curve.
Comment below with features you want/need a potential muscle rig to have.