Can I Mindslaver my opponent and then cast his Panglacial Wurm and have it come under my control?
No.

You seem to misunderstand what controlling another player does. It doesn't allow you to take their turn, or use their cards. It instead allows you to make all decisions they would make. This means that if they have a Rampant Growth, you can force them to play it. You do not actually get to play it; you would not get the land. They would still be the controller of Rampant Growth.

Similarly, any creature they cast (because you forced them to) would be under their control just like normal. Panglacial Wurms is no different. Its ability lets your opponent cast it at a time other than normal, from a place other than normal, but it still is just like any other creature spell once cast, and would enter the battlefield under the control of the player who cast it (which would be your opponent, even if your opponent is controlled by you).


  
  Controlling Another Player
  
  
  
    714.3. Only control of the player changes. All objects are controlled by their normal controllers. A player who’s being controlled during his or her turn is still the active player.
    
    714.5. While controlling another player, a player makes all choices and decisions the controlled player is allowed to make or is told to make by the rules or by any objects. This includes choices and decisions about what to play, and choices and decisions called for by spells and abilities.
  


And from the rulings on Mindslaver:


  You don’t control any of the other player’s permanents, spells, or abilities.


and


  You choose which spells the other player casts, and make all decisions as those spells are cast and when they resolve. For example, you choose the target for that player’s Shock, and what card that player gets with Diabolic Tutor.


From a strategic point of view, the reason you want to control a player isn't that you get to steal any permanent spells that they could afford to cast. It's that you could do things such as attack with creatures that will easily die, cast harmful instants and sorceries targeting themselves instead of you, maybe sacrifice all of their good creatures if they have an ability that allows them to do so, etc. So you can still do a lot of powerful things with the ability, but you can't just take their creature spells as your own.