I'm not sure why it should be complex. Set a date and anyone in the U.K. prior to that gets the same rights as now.
Nothing stopping them applying for citizenship. The system is already there I believe. Then hold dual nationality. Keep your rights and be a british citizen and then you can more easily insist on language requirements etc. British jobs for british people? Easy fix?
You can apply for British citizenship by naturalisation if:
you’re 18 or over
you’re of good character, for example, you don’t have a serious or recent criminal record, and you haven’t tried to deceive the Home Office or been involved in immigration offences in the last 10 years
you’ll continue to live in the UK
you’ve met the knowledge of English and life in the UK requirements
you meet the residency requirement
And you must usually have:
lived in the UK for at least the 5 years before the date of your application
spent no more than 450 days outside the UK during those 5 years
spent no more than 90 days outside the UK in the last 12 months
had settlement (‘indefinite leave to remain’) in the UK for the last 12 months if you’re from outside the European Economic Area (EEA)
had permanent residence status for the last 12 months if you’re a citizen of an EEA country - you need to provide a permanent residence document
not broken any immigration laws while in the UK
There are different requirements if your spouse or civil partner is a British citizen.