As a long time fan of a club currently in the ESL I would be delighted if our constitution were to be made up to a minimum of 20 teams for obvious reasons and don't see why some teams from mid Essex southwards in the Premier/Div 1 of the ECL shouldn't be invited to transfer to our league to make that number up.
After all there is much talk around non-league football regarding clubs being allocated to the league in their most suitable geographical location relative to the level at which they presently play to keep travelling expenses down to a minimum.
However there will alway be clubs on the fringe of two different league areas who may have to consider that they may be asked to move across.
Personally, I would think that travelling to grounds new can revitalise things rather than playing the same old teams and visiting the same old grounds. Although leagues with more than one division have a benefit of an automatic visit to one or two new grounds each new season after the relegation and promotion issues have been settled.