They must be adopted by a unanimous decision, and our correspondent says at least Britain is likely to oppose the changes.
Commission President Romano Prodi said urgent reform was vital to ensure that the biggest enlargement in EU history did not paralyse decision-making.
"The credibility of the Union depends on these initiatives," he told the European Parliament. "It is our duty to ensure that the expectations of European public opinion are met."
The changes must be agreed by December and completed before 2003, when the first of 13 countries hoping to join the EU could become members, Mr Prodi said.
Majority voting
A BBC correspondent says Mr Prodi's proposals are likely to prove controversial.
They must be adopted by a unanimous decision, and our correspondent says at least Britain is likely to oppose the changes.
Measures put forward include making majority voting the rule and reducing the right of member states to veto decisions they disagree with.
"In our experience, when we say unanimity it tends almost automatically to lead to paralysis," Mr Prodi said. "With 27 or 28 members this gets worse."
But he said unanimous decisions would still be needed for some matters, such as major tax or defence issues.
Fast-track integration
The other main commission proposal is that a group of member states should be allowed to go ahead with closer integration than others, creating what some observers have called a two-speed Europe.
Mr Prodi also wants the number of EU commissioners to be kept at the present level of 20, even after enlargement. Commission membership would rotate among nationalities.
The EU, which was created for six members, now has 15. It has opened membership negotiations with 12 nations, and has also accepted Turkey as a candidate.
Frontrunners, including Poland, Hungary and Slovenia, hope to join by 2003.