title_body: Soudwegian: A Swedish-German process similar to what happened to English and French? The idea is that the Swedes retain a small bit of what is nowadays German territory that they conquered in the 17th century. This is now independent territory, Soudway, with a language of its own named Soudwegian.
  
  Their vocabulary is basically German with a lot of phonetic changes, plus a huge amount of Swedish loans (and smaller but important contributions from Polish and French), while the grammar is very simplified, like Middle/Modern English as opposed to Old English.
  
  Is that a realistic choice, or was the grammatic simplification that characterised the process between English/French unique&mdash;not something that we should expect from other similar fusions?
upvoted_answer: Yes, that is a realistic choice in fictive works.
  
  Abstractly
  Nobody can be 100% sure of how realistic it is, and in conlanging&mdash;or even fictive worldbuilding in general&mdash;you may always be a bit unrealistic. Think about some fictive works or conlangs you know placed in another world; they make many assumptions that extraterrestrial life has two eyes, hair on their head, ~1m&ndash;2m tall, and their mouths have lips, teeth, an alveolar ridge, a glottis and a tongue. (Your idea takes place in our (but alternative) world, but the point of the example still stands.)
  
  Linguistically
  There could be many ways of forming such a language, of which the most realistic are (chiefly from Mark Rosenfelder's Advanced Language Construction):  
  
  
  A creole. A creole is, by definition, a pidgin which has native speakers. A pidgin is formed when two languages try to communicate, for reasons of trade or otherwise. If you read about Tok Pisin (etymologically "talk pidgin", a creole), you'll see a lot of simplified grammar (and, as you say, English is already quite simplified).
  From Swedish, but developing independently from main Swedish. As a native English speaker born in Holland, informal English I use to talk to my family is very different from main English. For example, I'd say *How late is it? (from Dutch Hoe laat is het?) by accident. Also, lots of sound changes happen. Even though I don't merge /w/ and /ʍ/, many people I know that used to be able to can't anymore, and my rhotic in both Dutch and English is [ʁ], even though neither Dutch nor the Scottish dialect of English has it.
  A combination of the two. After a period of isolation (independent development), the Swedes start talking more like the Germans (like creoles, but slightly different)&mdash;a combination of the two.
  
  
  So, even though it might be somewhat unrealistic, within the dome of fiction it isn't unrealistic enough to stop you.