Extending I-555 was discussed in another thread, but the result was the same.
There just isn't enough traffic west of Jonesboro to support it.
ARDOT upgraded AR-226 (Cash Highway) to 4 lanes to serve as the connector between US-67 (future I-57) and Jonesboro from the west.
While the concept of extending I-555/US-63 to a more serious route beyond Portia, west of Walnut Ridge would be nice for truck traffic coming up/down to Memphis, it probably won't get any serious attention until ARDOT gets funding to support a US-412 upgrade, which is on their priority route listing. When I-22 was finished between Birmingham and Memphis, this increased traffic to/from Atlanta. Every time they extend capacity to/from the southeast, it draws more traffic from the northwest (like KCMO).
As for improvements all the way to US-60 in Missouri, (which I think will become a true Interstate route someday as it follows National Priority Route #3), the best route would be US-63 from Hardy to West Plains, MO, but I can't see it being prioritized. ARDOT wants US-412 updated across the entire north part of the state and it doesn't have enough dough to finish I-49 as it is.
This would take traffic across north Arkansas to US-65 where they would go up the Branson route to reach KCMO. It's just not going to happen in our lifetimes unfortunately.