Beautiful work. Before I get into detailed comments, is it safe to assume you plan to pitch this to RTD at some point? I do not want to mention this post to friends at RTD (or show them myself) if you are planning to perfect this and then make a formal pitch (which I think you could and should).
I agree with Cirrus on some of the line names. And I would add to that that RTD is fairly married to the existing monikers used before/during Fastracks development, so except where they are nonsense, I'd stick with them. Besides, you're bound to annoy some nearby jurisdiction if you name them that way... So, that said -
Your "Golden" = "West" Line
Arvada = Gold
Flatirons = Longmont (that works)
Aurora is good, Boulder works too, Northside = North, but no biggie.
I'd call Arapahoe "Southeast" or even Tech Center or Lone Tree (worst option). Calling it Arapahoe is an obvious slight to Douglas County.
I don't particularly like it - I like your way, with or without any changes - but love it or hate it, I think RTD is going to stick with letters...
I would take a look at the latest RTD/Fastracks Service Plan (in the reports to DRCOG, easy to find online, or let me know). I know it'll clutter things up and I greatly greatly appreciate how you've simplified things. I love it. BUT...it's deceptive on the southern end. Specifically, your map makes me think I could have a one seat ride from Nine Mile to Union Station (not in the plan I don't think). Also from the Southwest through to 30th/Downing (I'd have to verify that, but it wasn't in the last service plan I looked at). Your orange line that currently ends at Nine Mile is supposed to extend to Florida (to 18th only), I believe. On that, I'd pull the downtown loop around (orange, possibly also red) to show the connection between 18th/California and 18th/Stout. The trains already switch their placards to southbound after the Convention Center, and it's become fairly standard to board on the California side and ride the loop. I don't see that changing.
As far as ongoing changes to Fastracks, of course your map assumes full buildout, which is still a vote away. But I would continue that assumption for now. No decisions have been made (final decision) that I know of on where to cut if that doesn't pan out and "permanent" cuts have to be found.
Is the station still at 38th/Inca? I though it had been moved to 41st/Fox?
A couple of naming conventions... entirely a matter of preference. But on the Gold Line, I think the "60ths" at Federal and Sheridan are unnecessary. That said, on the SW I would add C-470 to Lucent (good for the Highlands Ranch folk), and on the Airport line I would add "40th" to Airport. That's a major park-n-ride, and RTD will want it easily found. (I would add little PnR symbols to the stations that will have them (most), but Cirrus already addressed that). Last one, on the North Line, I would add "SH 7" to the 162nd Ave station - again, folks need that station to be easy to find by car. Globeville station might add "Coliseum" (and the Stock Show, as long as it's there).
I'd leave the fare zones off for now, knowing they have to be added eventually. But there is just no way on God's green earth that RTD is going to be able to retain the awful terrible zones we have now once all of this comes online.
Last thought, on the Boulder line (BRT), at the end, after the split (which you show very nicely), I'd pull it directly west, and then a jog northwest (like you have it) to the end of line in Downtown Boulder. In between, you have to add some reference to the University of Colorado at Boulder. Too many riders on that line not to tell folks boarding in Denver which line is the best way to get to CU!
Oh, I like the note under DIA. But maybe drop the "About," and just leave it at "40 Minutes to Union Station"... no transit agency wants to admit on its print map that it isn't always on time
All in all, fantastic!
Welcome home, JChurch.