The current system in place is, in my honest opinion, the best system to prevent tanking. What does tanking really guarantee you? A top 4 pick at best. It doesn't guarantee the 1st pick. It doesn't guarantee the 2nd pick. Heck, it doesn't even guarantee the 3rd pick. A team, even if they tank, can still fall out of the top 3, and pick 4th. While your suggestion,
@VF21, seems interesting,
I am not quite sure that it will really solve the issue at hand. Because, if a team knows for a fact that they will guarantee themselves a top 4 pick 2 years down the road, then they're going to tank right then and there.
Another idea to maybe toss out there would be...
If a team gets a top 3 pick in consecutive years, then they're ineligible for a top 5 pick the next year (yes, I did say top 5, and no, that is not a typo). OR, if a team gets the top pick in consecutive years, they become ineligible for a top 5 pick for the next 2 - 3 years.
I do think, however, that the current process of holding a lottery is the best system to prevent tanking.