The biggest single tax avoidance issue is builders who either build or renovate a property and then live in it just long enough to claim it as a personal residence and pay no tax. The solution is to allow only one tax free transaction every 10 years. 
The second biggest issue is with non resident investors who simply do not file any tax paperwork at all even when they invest in pre-sale condos and then flip the  assignments at a profit - the solution is to get proper control over foreign property ownership and to implement a system where sales by foreigners are subject to CRA approval prior to the release of funds. 
The third problem is with those that cheat the system by investing through relatives or who simply do not report profits from the sales of investment and second homes. 
All three issues should be the focus of the CRA who spend far too much time chasing unimportant  issues and not enough focus on this problem which costs the country billions of dollars each year.