I'd also look at a prospect swap with Niemela. He is progressing well for his age but the Marlies are loaded at RD (Myers, Mattinen, Villeneuve and maybe Timmins). If the Leafs make a call up I believe it's more likely Berube wants either of Myers/Timmins/Maatinen given their size and their age. Niemela is also 1 year away from waiver eligibility. He will need a spot on the NHL Roster by next season.
A prospect swap for Ryan Winterton could make sense for both sides. Seattle is loaded with U21 C's (Wright, Rehkopf, Catton, Molgaard, Firkus, Goyette). They currently have 1 RD prospect in the AHL. This would be a move from a position of excess for a position of need.
Toronto doesn't have many RD prospects but they also have very little C prospects and no RS C prospects. Winterton would be direct competition with Minten but he is 1 year older. He would be the only RS C on the Leafs depth chart right now and he's played a significant amount of games with Cowan on his wing. Assuming he can step into the NHL over the next season he can ease Cowan into the NHL the following season. Winterton is also local to Toronto.