Well, what would it be?
Let's discuss the possibilities:
1. 1500 and 3000 Meters - These two would, logically speaking, come as a package deal and replace the 1600 and 3200. A handful of states do it this way, mirroring the international distances. Arguing against the practice are the slightly greater difficulty of the 1600 and 3200, the convenience of the 1600 and 3200 starting lines, and keeping now nearly 40 years of records intact.
2. 5000 Meters - In terms of straight-up running events, this is the most logical addition out there. It is, after all, an international event in track and field. The 5000 meters doesn't seem terribly likely, though, as it has a lot of things working against it. One, it would add an enormous piece of time to the length of a track meet (though it doesn't seem you'd have to run it at every meet--a fact that collegiate and professional track meets have caught onto, but which doesn't seem to have any traction yet at the high school level). Two, we would be the only state in the nation to have 5000 meters. And, three, it's already tough enough to find young men and women who want to run the 3200 meters. It seems eight laps around the track already constitutes a serious thinning factor, and 12.5 laps wouldn't make recruiting any easier.
3. Shuttle Hurdle Relay - Sprinters have relays, so why not hurdlers? Most, though not all, schools have enough hurdlers to populate such a relay. But, if you think judging an exchange zone is difficult, think about judging the transition of hurdlers on this relay (at least you only have four lanes to deal with, though). If Colorado were to add this relay, we would join Iowa and West Virginia as the only states contesting this relay at the state meet level. Presumably, as a state event, you would have to have heats against time rather than a single, unified final.
4. Distance Medley Relay - Of all the baton relays Colorado doesn't have, the distance medley seems the most likely one that could be added. There are a couple of considerations in its favor, most notably that it would help to balance relays out a little between sprint relays and distance relays. And, the relay enjoys a presence at the collegiate level, though only in the indoor season. We would have to concede, however, that a lot of schools would have difficulty populating this relay, and especially so if we had both this relay and the 4x800. Realistically, it's either this relay or the 4x800, and the 4x800 has the virtue of weighing each leg equally, which the DMR does not. For those wondering about the composition of the relay, it goes 1200-400-800-1600.
5. 400 Meter Hurdles - I never watch this event without wishing we had it at the high school level here in Colorado. Five states already use the 400 meter hurdles, and we could be the sixth in a heartbeat. Please don't tire me with the spiel about it's too long of a race for high school hurdlers. Every USATF club meet every summer deep-sixes that myth. I've never seen an athlete drop out of the 400 meter hurdles from sheer exhaustion. This event would, of course, come at the expense of the 300 meter hurdles. But, the people assigned with transporting blocks from starting line to starting line would applaud the change.
6. 2000 Meter Steeplechase - This event has serious crowd appeal! People love to gather by the water jump and watch this event. And, Colorado already has a serious presence on the all-time national leader boards for this event. The event combines the disciplines of hurdling and distance. There is, however, one very large and enduring problem--only one high school track in Colorado has a steeple pit. For those wondering about the cost of the event, I'm guessing the complete set of barriers altogether run about what two high jump pits would cost, and considerably less than what an entire pole vault setup costs. But, it is still expensive, and especially so when you ponder the cost of adding water jump pits to tracks all around the state.
7. Javelin - If you don't wish we had this event in Colorado, you have probably never watched the event. It is a fun event to watch. Two obstacles stand in the way of this event. The lesser of the two obstacles is the safety factor. But, arguably, the javelin is safer than the discus. The pointy end of the javelin troubles a few folks, but even if I was stupid enough to be standing in the sector without looking, I think I'd still rather take my chances with a javelin than with a 1.75 kg discus. Either one, of course, could kill you. That's why they have the sign at the Air Force Academy, "Heads up or heads off!" I suppose it's not very politically correct, but it gets the point across. And a final note on the safety of the javelin relative to the safety of the discus: the javelin does a much, much better job of staying in the sector than the discus does. In short, it's easier to control the safety issues with the javelin than with the discus. The greater obstacle for the javelin is the real estate factor. You have to have a place to throw the javelin and the infield isn't really a viable option at high school tracks these days. 17 states have said "yes" to the javelin, including three states that border Colorado: Kansas, New Mexico, and Utah.
8. Hammer Throw - This is another event this is great fun to watch. It is, however, unquestionably the most dangerous of the throwing events. You can dodge the ball and still get maimed by the chain. And, this event requires an industrial-grade cage. The cage would be a serious, serious investment for high school level programs. Despite the obstacles, however, the event is contested at the high school level in Rhode Island.
9. One Mile Racewalk - We could link arms (though they would probably have to be high and tight) with Maine if we had this event. But, please, no. First, racewalkers find it pretty nearly impossible to comply with the published rules of their own discipline. Second, I have no desire to watch rolling hips of both genders for even as little as four laps. If I wanted to stare at people walking for minutes on end, I'd find out when the local senior citizens are doing their weekly mall stroll and take a seat at the fountain.