I believe that putting a team in this new arena in Idaho would help sure up the mountain division, which is something the ECHL wants. Easier travel for Idaho, Utah, and to a lesser extent, Rapid City is important to keeping the westernmost teams, two of the longest continuous ECHL teams, sustainable.
For other expansion, Clarkesville, TN is looing really good to the ECHL. IIRC, the arena would be fairly larger than a typical SPHL arena, making it the perfect fit, both size wise and geographically, for the ECHL to expand.
Next, I have not seen it talked about too much, but Savannah, GA is building a new 10k seat stadium with ice making capabilities. While it is prime SPHL territory, I think the ECHL would be making a huge mistake to not attempt to grab this market.
With the last two, you always have to worry about the FPHL getting in the way as they expand farther south.