Interesting idea, but not likely feasible. RNAs can not freely cross cell membranes to enter target cells, so you would have a major delivery problem, especially when coupled to a hypothetical "collection molecule" such as a short synthetic polymer or engineered protein.

Moreover, mRNAs are degraded by cytosolic RNases. Any modification that prevents RNA degradation is likely also going to impede interaction with and translation by the ribosome. So there is no simply way to modify mRNA to render it impervious to degradation without destroying its functionality.

For these reasons, gene therapy focuses on DNA-based methods, since it is much simpler to engineer stable gene integration into a host than to try to design a DNA-independent, self-sustaining RNA-based gene.

For your approach to work, at a minimum, you need to conceptualize a way that your mRNA can self-replicate.