denver - no active conversations as far back as i can remember. it's a looooong distance and very few trip generators in between. probably more realistic to connect via omaha (the california zephyr and southwest chief have similar arrival/departure times in KC and omaha, making a thruway bus a near-term option) or one of the southern colorado towns served by the chief. kansas would need to participate since the distance is under 750 miles, since federal law now prevents amtrak from fronting the full cost of routes shorter than that. kansas has been most concerned with keeping the chief running through the state (it was at risk of being rerouted through texas).
omaha - no active conversations, but there was a state senator who was lobbying to connect KC to st. joe, which would use the same UP (?) tracks up to omaha. jefferson lines services this route today, so it would be possible to have amtrak contract with them for a "thruway" service to build ridership. "feeder" bus service to st. joe was also considered in a draft state rail plan a few years ago, but, alas, not to omaha.
springfield - modot studied
STL to springfield in 2007, but the existing tracks needed a major upgrade and the terrain east of springfield makes for a circuitous (thus slow) route.
joplin - presumably a route to springfield could continue on to joplin and eventually OKC, but that has never been seriously discussed.
on an extreme far side note, if a hyperloop is ever built to connect the coast they would likely make at least one midwestern stop. chances are that would probably be chicago though.