Just imagine the completely different world today if Russia were a part of Free World, member of NATO and a prosperous democracy, like Germany.
Was it ever discussed, and if so, why wasn't it implemented?
Short answer: the situations in 1940's and in 1990's were significantly different. The scale of the required aid and nature of reforms was incomparable. There were talks of increased aid in exchange of massive reforms and nuclear disarmament, but they appeared to be unsuccessful.
The major plague of Russian economy was not lack of finance. Instead, the structure of production was based on extraction of mineral resources and everything directly related. So filling it with money would not help establishing a better structure of production;
The items above are greatly summarized in early "Chicago Tribune" article of 1991:
Even if the United States were in a position to underwrite generous aid for the Soviet Union, as it did for Western Europe after World War II, there are ample reasons why it shouldn`t. Domestic woes, the U.S. budget deficit and the undetermined direction of Soviet change suggest caution.
One of the key goals of Marshall plan was containment of Russian expansion. In other words, it was a strategic defensive plan aimed to strengthen economical and political security of European countries of late 1940's. And there was no Russian threat in early 1990's.
Many people see Marshall Plan as a purely economical program. Others also think it was act of generosity. Churchill called Marshall plan "the most generous act in history." However, it wasn't.
The Marshall Plan was the Truman Administration's plan to rebuild war-torn Europe in order to aid building strong, self-sustainable economies, facilitate global trade and free markets, encourage European peace, and prevent the spread of communism. George F. Kennan formulated the policy of “containment”: "A long-term, patient but firm and vigilant containment of Russian expansive tendencies".
Some political forces on Russia (like the first government of Yegor Gaidar, Nov'91 - Apr'92) attempted to negotiate "Marshall plan" in exchange of massive economic reforms, deregulation of prices, and nuclear disarmament. Unfortunately, these forces were not in power for enough time and they couldn't convince the U.S. officials that such aid would be used properly:
[Aven and Kokh] slip in an interview with the 85-year-old James Baker, US secretary of state from 1989 to 1992. Aven presses Baker hard on why the US didn’t give Russia more financial aid, saying it could have allowed them to embed liberal reforms. Baker didn’t think so, and still doesn’t. — Financial Times
I could not find this interview in English. Here's the Russian version at Forbes.ru.
As a result, at the G7 meeting in Houston (1990), it was decided that offering the Russia official aid would not help. The task of providing aid has been authorized to the IMF, which usually operates with "standard-sized" crises, but is not adapted to resolving consequences of collapse of entire economies.
A decade and a half after, Yegor Gaidar, in his book "Смуты и институты" ("Time of Troubles and institutions"), recalled:
«План Маршалла» был реализован потому, что его выработало и провело в жизнь руководство страны, вышедшей из Второй мировой войны. Оно понимало, что столкнулось с новой войной – холодной. Это позволяло консолидировать усилия, мобилизовать финансовые средства, сделать программу помощи важнейшим приоритетом американской политики. В начале 1990-х годов ситуация была иной.
"Marshall Plan" was implemented because it was developed and put into practice by a country who survived World War II. They understood that they faced new war — the Cold War. This made it possible to consolidate the efforts, mobilize financial resources, and help make the program the top priority of American policy. In the early 1990s, the situation was different.
There was no Marshall plan for the 1990s Russia for the same reason there was no Marshall plan (or other significant aid) for Brazil, India, China, Indonesia, the countries of Central and Latin America, and all of Africa.
That reason is that the US government helps countries only when doing so provides a clear benefit ... to the US elites. As explained in Part 5 of the earlier answer, rapid development of Western Europe (and to a lesser degree South Korea, Japan, Taiwan) was meant to help contain the spread of communism.
The US elites however have no need for competition from a strong Russia. On the contrary. The US elites need a weak Russia that would be nothing more than a source of cheap natural resources similar to the Middle East, Africa, or Central and Latin America.