It will never happen and would be a financial disaster if it did. Firstly some EUFA/FIFA members have long resented the fact the 4 home nations each have a vote and have wanted the UK to be only allowed 1 vote.
Secondly and perhaps more importantly imagine Inverness playing in the same league as say Exeter and Plymouth, you are talking of a 1200 miles round trip. Sods Law they would be drawn together in both cups as well.
And if you just took Celtic and Rangers out of Scottish football, interest in the Scottish league would collapse, and Celtic or Rangers would lose almost certainly their Champions League football, which currently they are always virtually guaranteed.