Barfield's understanding of Romanticism as an uncompleted destiny is of primary importance to an integrated view of The Inklings. Barfield articulated this very clearly in Romanticism Comes of Age (1944) as well as subsequent books; but none of the other Inklings seems to have understood or been-persuaded-by the argument.
(Probably they were put-off by the fact that Barfield drew much of his understanding from Rudolf Steiner, and referenced Steiner's work frequently. Because although Steiner was certainly a genius of world historical importance (Barfield seriously and coherently compared him with Aristotle); he was also wrong about most things, most of the time - and in a peculiarly gratuitous and overwhelmingly prolix fashion.)
In a nutshell, Barfield saw Romanticism as a necessary but unfinished, indeed corrupted, development of human consciousness. Therefore, he saw the only viable option for modern Man as the completion of this change.
What happened to Romanticism, according to Barfield, was that it arose in the late 1700s and initially was taken forward very promisingly in practical terms by Wordsworth and Blake in England and Goethe in Germany; and was very fully theorised by Coleridge... but that it was soon diverted into modernism - that is metaphysical materialism/ positivism/ reductionism/ scientism - in the forms of varieties of atheism, political radicalism and sexual revolution.
By contrast, although both Tolkien and Lewis embodied Romanticism in much of their work, and were anti modernism in all the forms described above - they advocated and adhered a return to pre-modern states: Traditionalism.
In sum, rather than moving-through Romanticism to a new form of consciousness (along the lines theorised by Coleridge); Tolkien and Lewis attempted to fuse Romanticism and Tradition, or perhaps to use Romanticism as a means for returning to Tradition.
The completion of Romanticism is therefore still an uncompleted project! Indeed, the whole issue has proven to be very difficult to discuss at all. Coleridge never succeeded in making himself understood by anybody! - arguably until Barfield himself brought together and interpreted many scattered passages (e.g. especially in What Coleridge Thought, of 1971).
It is no mystery to me why the project of Romanticism remains uncompleted - to complete Romanticism requires:
1. A rapid (not incremental) and wholesale (not partial) replacement of fundamental metaphysical assumptions concerning the basic nature of reality. And...
2. This task must be done actively, voluntarily and explicitly by each person as an individual.
By contrast, the modern prevalent perversion of Romanticism (i.e. atheist, materialist, leftism) was introduced incrementally, unconsciously, by mass influences and in a top-down fashion; such that the whole system of modern Western thinking is commonly unrecognised and denied, is incoherent and self-destroying, is dishonest and relativistic.
Could Tolkien and Lewis have followed where Barfield was leading? It is hard to imagine - since they would need to be convinced of the impossibility of Traditionalism and also of the possibility, and goodness, of completing Romanticism. They would need to acknowledge that Romanticism was, in its original impulse, not merely a 'reaction' to industrialism and materialism; but an embryonic new form of consciousness. Romanticism-completed would (even without Steiner) also have struck at the particular self-definitions of Tolkien and Lewis's different, but in this respect similar, definitions of Christianity in terms of creeds, institutions, and authority...
Tolkien and Lewis would therefore surely have found it difficult to distinguish between Barfield's suggestions for a completed-Romanticism and the modernism they opposed root-and-branch. Nonetheless, we can - looking back on the Inklings and perceiving them as a spiritually-coherent group who themselves had a destined role to play in the development of Western consciousness - ourselves do the work that Tolkien and Lewis could not, and would not have done.
Which is to complete Romanticism with the help of their imaginative literature.