I have not heard of any such theory becoming a credible candidate for acceptance, although I see no logical contradiction in such—my impression is that discovering a time-varying term would be as surprising as discovering energy is not conserved. For fairly fundamental reasons, actually.
Note that in GR defining energy consistently is tough. Doing it so it is globally conserved is even harder. We only really have local conservation, and the changing background of GR in cosmology is in some sense effectively the same thing as changing physical law.
I have not heard of any such theory becoming a credible candidate for acceptance, although I see no logical contradiction in such—my impression is that discovering a time-varying term would be as surprising as discovering energy is not conserved. For fairly fundamental reasons, actually.
Note that in GR defining energy consistently is tough. Doing it so it is globally conserved is even harder. We only really have local conservation, and the changing background of GR in cosmology is in some sense effectively the same thing as changing physical law.