Widening the existing SH-50 crossing seems like the most sensible option.
I'm not sure how much that would actually help, though. Someone correct me if I'm wrong, but I don't think the 2 lane bridge out there is inadequate for the traffic levels there. A large part of the US-93 traffic is local traffic (for commute, shopping, etc.) between Twin Falls and Jerome. As well as other towns in the region like Wendell and Shoshone. In addition, Boise to Twin Falls traffic is not insignificant, especially factoring in people from Boise driving through Twin Falls to get to Nevada on US-93. All these use cases would be ideal for a western bridge, especially for Boise to Nevada traffic bypassing the city (a win-win, less traffic in the city, faster travel for people bypassing). Widening ID-50 would do nothing for the Jerome to Twin Falls traffic which is the biggest contributor. I'd imagine that Twin Falls to Burley (and beyond) traffic is split between US-93 and ID-50 and is a smaller percentage of the overall traffic.
That said, it would be the cheapest option. But seems like widening it would be kind of a waste of money.
Also, thanks for posting this here. I saw an article I think from KTVB on Facebook, and it was almost completely devoid of any useful details.