I've been kicking this idea for a long time. I might eventually get around to making this into a TL although it's
sort of been done before. Basically the premise is if I had no knowledge of what Middle or Modern English sounded like, but knew Old English, Dutch, Frisian, and the other Germanic languages, what might we guess *English would be like?
Other posters touched on grammar, but I will mention vocabulary. One measure of how similar languages are is called lexical similarity, which typically uses something called the
Swadesh list. The Swadesh list contains about 200 words that are very common across languages. Lexical similarity is calculated by dividing the number of similar words by 200 (see page 107
in this book). Now for all other Germanic languages, lexical similarity is generally pretty highly correlated with geographic distance. No big surprise that Dutch, Frisian and German are all quite similar. The same can be said for Danish, Swedish, and Norwegian (Riksmal). And Icelandic and Faroese are unsurprisingly big outliers. Using this same formula, you'd expect English
a priori would be pretty similar to Dutch and Frisian considering the short geographic distance involved. But it isn't. In fact, English is no more similar to Frisian than it is to other Germanic languages and only slightly closer to Dutch than to the other Germanic languages.
One of the things that makes English weird is that even its basic vocabulary is different from other Germanic languages. Some of this can be accounted for by the Norman Conquest, which includes words like forest and animal. But there's also a lot of stuff that just can't be accounted for. Take the words bird and dog. Nearly every other Germanic language retains the
proto-Germanic terms, which are cognates of the English words fowl and hound. Old English used the perfectly normal
fugol and hund. However, these terms underwent a semantic shift during the Middle English period. It's not even clear where the words bird and dog came from, as they have no cognates in any Germanic, Celtic, or Romance language. English is just different for no good reason.
Now without the Norman Conquest disrupting things so much in England, maybe we would still call birds fowl and dogs hounds. If we were conlangers in an alternate universe who had no knowledge of Middle or Modern English, we probably wouldn't end up with a language that sounded like English. *English would likely be more similar to Old English than it is now (but not exactly the same!). There are also other things about English that seem improbable to me, like the survival of the "th" sounds, which died out in all other Germanic languages except Icelandic (though Celtic Welsh has it, which may have helped its preservation). Even many English dialects have dropped it. So it's entirely plausible to postulate the loss of that phoneme in *English judging by the fact it was lost everywhere else in Germanic Europe except in isolated Iceland.
I won't wade into the debate about the loss of the case system, but if it were me, I'd assume that *English also would lose its cases since Dutch and Frisian also lost them and the case system was preserved only in the northwest (Icelandic/Faroese) and southeast (German) of Germanic Europe. That said, there's a fair chance of cases being retained in Scotland, due to its proximity to the Faroe Islands (especially if a
Norn-like language develops there).
Even with butterflies alone, English might end up sounding very different. Consider this
real-life example of a highly divergent dialect of English. I certainly can't understand it without a translation, even if I can pick out a few words here and there.