Improvements to the Cardinal would include making the train daily to boost ridership, for starters.
In addition, the Hoosier State would be moved to a schedule independent of the Cardinal and go to twice daily service between CHI-IND,
and extend both of those round trips in two different directions to two different cities. One of the Hoosier State round trips would be extended south to Louisville and Nashville as a day train,
and the other would extend to Cincinnati, giving that city daylight service.
AND split the Cardinal at Indy to send a section to St Louis. (This possibility was raised in the PRIIA study of the Cardinal back in 2009 or so. It was not in the purview of that report, but was deemed worth further study.)
AND invest in upgrades to the Indy-CHI route. To start, Indiana paid for a study that proposed some $225 million to take 29 minutes out of the run time from Indianapolis to the Illinois state line.
The faster run would allow the Cardinal an earlier arrival in CHI (now 10:00 a.m.) and an earlier return to Indy (now midnight). The faster time on this segment would mean faster trips to Louisville and Cincy as well. Of course, it would probably be possible to cut at least another half an hour from the Cincy-Indy segment or more. Again that would allow an earlier arrival in CHI for the Cardinal (or a later morning departure from Indy).
Up until now, Amtrak's chronic shortage of equipment has prevented even much speculation about adding frequencies Indy-CHI or extending the Hoosier State corridor to Cincinnati/Louisville. But within 3 or 4 years Siemens will be delivering new equipment for the Midwest corridors. Adding another dozen, or two dozen, cars to that order should be simply a matter of money.
Of course, there's no money now, but things can change quickly, and not always for the worse. A political upheaval or/or another deep recession could lead to Stimulus-type funding to pay for the needed infrastructure upgrades.