I would think US-151 between I-39/90 and I-41 would become an interstate before WI-29. But even that's a long way off, and I don't think WIDOT seems very interested in 3dis since there are numerous highways that would qualify for them now.
At this point I think WI-441 would be the leader in the clubhouse for US or State highways in the state that could receive an interstate upgrade.
If an interstate was to be extended then maybe I-41 extended to Abrams, or 39 up to US 8 or near it......maybe
I don't know what is stopping WISDOT from slapping the red and blue shields on WI-441. They would have no problem getting federal approval to make it an Interstate. They wouldn't even have to change the number.
Same reason they didn’t try to slap a US shield on it. First, it’s more of a loop route than a legit bypass, second, it’s more of a local route than anything, and third, they just don’t seem interested in spending time and money on it. I can appreciate that.