I've played also over 20 years with my toy-model of "cosmic buoyancy" nowadays newly named "vacuum buoyancy" which is based on hyperfluid with positive/negative mass and inertia of gravitational changes.
The ideas Sydney linked come close to my buoyancy-toy-model.
In case it can help you, I'm sure that this alternative you do not know it:
ALL SPECIAL RELATIVITY EQUATIONS OBTAINED WITH GALILEAN TRANSFORMATION & EINSTEIN FIELD EQUATIONS OBTAINED ONLY WITH GAUSS CURVATURE AND ZOOM UNIVERSE MODEL CHARACTERISTICS ( ZOOM THEORY )
The problem of nature of gravity is linked immediately with problem of time. We must consider the problem of time in detail. If we do not consider the time only as uniform flowing we open the new world. The time in this world can flow uniformly, non-uniformly, the time can be stopped and flow in opposite direction --- from the future to the past. Assuming this position, we obtain that gravitation can be as the energy of attraction so energy of repulsion. These results can be obtained from the space-time described by de Sitter metric with positive lambda. Considering Schwarzschield metric in non-empty space (liquid sphere with constant density), we obtain that this space-time transforms by some conditions to the de Sitter space, filled by vacuum in the state of inflation. We obtain that the time inside and outside sphere flows in opposite directions. Sign of time changes at the surface where the time is stopped. With a position of a real observer Newtonian gravitation of attraction is linked with positive flow of time, and non-Newtonian gravitation of repulsion is linked with negative one. Gravitational force of repulsion is proportional to the distant from gravitating mass. Schwarzschield solution in empty space is the generation of Newtonian theory which is space section of Schwarzschield solution. Gravitational field is the field of repulsin or attraction in depend of distribution substance inside gravitating mass.