My $5 worth is they don't need a new rule to handle this situation. They just need to tweek the existing rule. First if a fielder is behind the bag that should allow his protection because that means the double play was so quick the relay is being thrown before the runner can get there OR it's act like a first baseman and stretch while touching the bag. In that case there's no chance at turning the double play but you shouldn't be able to slide over the bag and take out a fielder.
Change the "has to be able to touch the bag" aspect. How many times do we see a runner slide at a MIF who is facing the OF instead of actually trying to reach the bag? This needs to be the runner actually has to touch the bag. This should pull the runner back into the basepath a little bit more.
Last - common sense needs to allowed to make judgements. In this case common sense tells you that Utley went out of his way to take out Tejada. Doesn't matter what intent is because that's so hard to figure out. The intent was to break up a double play although as PG said Utley should have realized there was no need to break it up.
I don't want to see the high school rule of only going into the bag. The existing rule is fine with small tweeks.
Also, I don't care how "legal" this slide was - I think any umpire in MLB would have been fine if they called interference because that's what it was - interference.