What amendment would you add to the U.S. constitution that would limit the federal governments powers ?

The Amendment to make lobbyists and lobbying activies illegal. 

Require presidential candidates show their tax returns and take a mental examination to exclude people with malignant personality disorders.  Candidates should not have a history of running every business they ever ran into the ground.  They should not have boasted about grabbing pussy and should not be indebted to foreign dictators.  They should be required to read beyond the sixth grade level. They should be hooked off the debate stage if they tell more lies than any candidate in the history of the USA.  They should be able to identify at least one article of the constitution.

the limits already exist in the Constitution, but the document is only as good as the people who ENFORCE it. if politicians ignore the rules of law, and aren't brought to justice, no amendment is going to make a bit of difference. they'll just ignore it too. it's up to the people to demand their politicians obey the law. 

Abolish the 2nd and allow limited hunting gunz.

Read the 10th amendment. It already limits the pwers of the federal government