I think someone did something like that. And the result was absolutely terrible.
Daniel Deronda is absolutely my favourite novel. And I don't think at all that the 'Daniel Deronda' bits are an intrusion on a novel about Gwendolen. I think that they elaborate on, and provide a wider cultural context for, the 'saviour' or messiah role that she has created for him in her life.
I haven't read any commentary on the book, but I think this strange kind of saviour role that Daniel has for Gwendolen is absolutely about Gwendolen not about Daniel. Her 'redemption' via him is absolutely her own work, and his life outside his relationship with her magnifies the reader's understanding of his passivity and Gwendolen's authorship of her own life
I think Eliot was reflecting on religion, on the notion of coming to self-knowledge via the reflective medium of a god or messiah, and Daniel is a proxy for religion in the story. Calling the book 'Daniel Deronda' instead of 'Gwendoln Harleth' feels a bit like a nod to the contemporary Feuerbachian critique of religion -- the claim that when we talk about God we are actually talking about ourselves, projecting onto another entity the drama of our own self-realisation
Gwendolen knows nothing much about Daniel. She projects massively, using him as a kind of canvas on which to try and paint her own redemption. In that respect he is a bit like the more modern figure of a psychoanalyist, who helps a patient to reach self-knowledge via the much-vaunted 'transference', and I often wonder what Eliot would have thought about psychoanalysis.
He is her tool, a catalyst, and the 'Daniel Deronda' bits of the story seem to reflect on what it is to be trapped in this role of being a device for other people's redemption. (I haven't read it for decades, but I do seem to remember that all the confining circumstances of history and destiny kind of trap him into taking a certain direction, like Christ being funnelled by his own destiny into the passivity of suffering and death.)