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One possible problem of your proposal could be if the team "winning" the toss was known weeks before the game this could give the ground staff the chance to prepare a pitch which would be to the advantageous to the home team.
Another alternative would be having the toss before each game, as now, but have a rule that a team can only win a certain numbers of tosses in a series e.g. 3 in a 5 match series.