For a window onto how artists in the past dealt with the problem of being modern, read Giorgio Vasari's Lives of the Artists. Consider that the century between Giotto's work and Masaccio's is a third of the time between Masaccio and Tiepolo--and those three centuries are twice the time that separates us from Tiepolo.
What makes a fresco "Modern"?
Pietro Annigoni's frescoes are clearly modern in spirit; often tennebrist, his religous works speak to the anxiety of faith in the modern world, and while rooted in Realism betray influences from both Surrealism and Expressionism. Annigoni, it seems, strove to make his work recognizably Modern even as he dealt with ancient stories and timeless themes.
David Mayernik's work is more overtly rooted aesthetically in the Old Master tradition, and yet his interpretations of classic themes are conspicuously modern, or informed by modern scholarship and intellectual culture. For him the language of his art is the vehicle for the paintings' meaning, and its timelessness is important both for its beauty and its ability to speak across time.
There is no compelling reason for modern frescoes to abandon the narrative structure that generated fresco painting in the first place; indeed, fresco existed for millennia precisely because it allowed walls to "speak," and the fact that we can still hear what they have to say is a result of their artists' concern for being articulate. Abstraction, for example, seems wholly anachronistic with respect to the raisons d'être of fresco. For frescoes to be modern, like those of Annigoni or Mayernik, they must acknowledge the modern condition, but they need not reinvent the language of narrative painting. On the contrary, they can draw on the resources of an incredibly rich past to construct a beautiful and eloquent present.
David Mayernik, copy after Tiepolo's fresco of Sarah and the Angel, Archbihsop's Palace, Udine