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There is some historical usage of diacritics in English, like naïve, résumé or even façade. I've been once told that these are used to mark a different spelling, and it may be used like in coöperative instead of co-operative to split the 'oo'.
I've checked with Wikipedia:
The main use of diacritical marks in the Latin script is to change the sound-value of the letter to which they are added. Examples from English are the diaereses in naïve and Noël, which show that the vowel with the diaeresis mark is pronounced separately from the preceding vowel;
Such a situation frequently happens in word formation, where joining a prefix that ends with a vowel and a word that starts with one. For example: coinduction, preimage, prainvention, etc. Normally one could use a hyphen, but that might be undesirable, and without anything words like bioracle (joining bi- and -oracle, whatever that would mean) could be understood as bio-rackle (I'm sure there are better examples). For example, co-op is sometimes written coöp, stretching it, bioracle could be written biöracle (I admit, it does not look good).
I know this is not a real issue, as most prefixes are known and it is possible to recover the source given context. However, technically, is this correct? To give a more concrete motivation, would it be plausible that in some future version of English (e.g. in a novel) such use of diacritics would be more frequent?