An Ada-Canyon based district
Idaho redistricting is approaching; the reapportionment commission is expected to be called together on June 7, with meetings to start soon after, with an eye toward developing plans within three months.
Unlike Oregon, where there's already some partisan battling going on, and Washington, where there soon will be, Idaho's maps are likely to have less at stake. The Republican advantage around the state is so sweeping that the only real concern will be incumbent protection. The Democrats are vulnerable in a few places, but that mainly translates to whether the Boise area will continue to have four legislative districts in which Democrats can realistically compete, or whether the number drops by one (or possibly two, if Republicans get very creative).
The two congressional districts are almost sure to look very much as they have for a generation, with Ada County divided between them. That's not a requirement, and it could be done in other ways. But it probably won't be.
Submitted as an example of why: The map here, drawn with the state's publicly-available and easy to use Maptitude software. Suppose you start with the premise that Ada and Canyon counties will be united in a single congressional district; what does that imply for what the districts would look like? This map shows you.
The problem is that there's only enough population for two districts, which means the other district would have a population base in northern Idaho and another in eastern Idaho, with no useful linkages directly between them - the Boise area is the real link between northern and eastern Idaho. You could do it, but you'd have a hard time justifying it to the people in the north or the east (or the unfortunate representative trying to cover it all).
The software, however, is highly recommended. Visit and have fund with it, especially with legislative districts, which is where most of the attention is likely to go.