The 12 SPL clubs met again to discuss the issue at Hampden yesterday with more talks pencilled in for November 1.
The signs are that clubs are veering towards a 14-team league which retains the split but removes the fixture anomalies and inequalities.
But no vote was taken and the debate over the size of the top flight and the fixture calendar will continue.
The situation is complicated by the fact that former Scottish First Minister Henry McLeish is due to deliver the second part of his report into the game before the end of the year, this time focusing on the professional game after previously exploring the grassroots scene.
Doncaster has been keen to consult with the other governing bodies and he admits there is little prospect of change being implemented next season.
He told BBC Scotland: "I think it would be ambitious. I am always hopeful that we can achieve things as soon as possible, but there is an awful lot of work to be done and we are only part way through the process.
"There is a general recognition within Scottish football that something needs to change.
"It is vital that what we do is to achieve something that improves the whole of Scottish football going forward and is not just considered piecemeal.
"We are trying to put forward a whole package of measures that we can then put to the clubs and vote on, because ultimately we need 11 hands going in the air to achieve any change, so that consensus package is what we are working on."
Doncaster added: "The format of the top league is something that is obviously a key part of our deliberations, but it is only part.
"What is important is that we put forward a plan for the whole of Scottish football and we do so with the best interests of all clubs at heart.
"In terms of per head of population, we are the best supported league in Europe.
"We have a lot to shout about, but at the same time there is a general view, probably rightly, that 12 teams may have had its day."
SPL managers have long argued for a different structure and Hearts boss Jim Jefferies is keen to move away from a situation where teams play each other so often, although a 14-team league would mean some clubs still facing each other four times.
Jefferies said: "We are always trying to get away from playing each other four times.
"Whatever size the league is, if it means playing each other less times, I'll be in favour of it."