The public transportation system within the City and County of Denver, absent bus, is almost totally non-existent, except for a few stops inside of the city limits along the currently developed Fastracks routes. There will also be a few more added as the East Line, North Line, and a couple of others of lines are added. With all done in total, there still won't be enough to actually meet the public transportation needs within the City, and least from my POV.
As mentioned, Colfax, Broadway, Colorado Blvd, maybe Federal Blvd, and possibly a few other thoroughfares are definitely suited for more than just buses. It will probably require studies (as if there hasn't been enough already) to get a system coordinated within the City. I know that Streetcars seem to be the cost-effective preference, but going underground is probably a better solution, even though at present costs would appear to be prohibitive for these at this time. But..... in terms of speed, efficiency, safety and even desirability, this is what the City needs to be considering.
With that, Denver has a unique situation in terms of its physical/geographical composition. We have a downtown area, and there we have offices, the 16th Street Mall, a major league ballpark, theatre and live entertainment, museums, architecture (both old and new), fine dining, and other amenities consistent with a DT area. However, what is lacking DT is major premium shopping, and the bulk of which in the City is located at Cherry Creek. This breakup of geography, I feel, breaks up what allure the City has as being many things to many people, and being an attractive regional destination for any of those who would want to take an all encompassing (if that's the correct way to phrase this) adventure to the City. The great cities in the world - New York, London, Chicago, I could go on - all have the same DT amenities that we have as I've mentioned above plus premium shopping located within their DT areas..... but DT Denver lacks the premium DT shopping.
So on the subject of intracity transportation here, one of the first goals would be to bridge the geography of DT and Cherry Creek. A subway that would take some three or four minutes at most from Union Station to Cherry Creek (maybe with one stop at most along the way, but no more) would, I think, basically make DT and Cherry Creek virtually the same geography. It would be a boon for both City residents of DT and Cherry Creek, as well as more attractive for those out of town visitors to the area much more appealing, as such subway would virtually make DT and Cherry Creek one and the same. And the fact that the premium shopping in Denver is not located DT, but in Cherry Creek, and the two are too close in proximity for many premium retailers to have locations in both areas, is also in a way a handicap.
So that's just my thoughts as to where to begin with intracity transit in Denver. I'm sure that I'll hear from a lot from those that disagree (and I would welcome it) but to have the image of DT Denver having its premium shopping in the DT area, a high speed underground transportation system connecting DT and Cherry Creek could do wonders in in this direction.