Thragtusk was a juggernaut while in Standard, and is now a Modern darling with obvious eternal demand.
This is a highly splashable and efficient card, and has a lot of of synergies.
Only a single printing means that the price here is on a fine trigger. A big performance in a prominent event may arouse new and old interest in the card. It looks like the stage is setting for this to happen, after a deslackening starting in mid-summer 2015. This is one to watch closely, and one on which to get a base coat going (a few play copies) before it starts cresting.
These are on their way to $8 in 3-6 months, $12 within a 18 months.
Unfortunately, there is also a seemingly impending reprint looming. And a reprint will strongly suppress the price in this case (single previous printing). These could show up anywhere. Consider these a short-mid-term pickup.
Foils are already pretty strong, at $15. Foils could see $30 within a year, but I would focus on non-foils for proportional gains and liquidity. A single FOIL might a good long-term hedge against a non-reprint.