What happens if I don't amend my tax return to include my w2?

Your W-2 is provided to your, and electronically a month later, with the IRS by the employer. Always. The IRS computer will eventually match your return to what has been reported to them (on 1099, W-2 and various other sources), see something amis, and bill you --plus penalty and interest...and likely review other parts of your return as it seems you aren't to be trusted...for that and priomand future years.