Going on your presumption, that money is essentially no object, I'm going to also presume that the unions can deliver on the warm bodies needed for construction (they certainly have made enough noise at Metro Board meetings about that). So I believe the following projects are possible by 2020 (with the rationales):
Crenshaw/LAX: LPA is the target for year's end. That makes EIR/EIS possible by 2012, and Eastside Gold Line took about eight years to build, including underground.
Expo Phase II: Expo Authority is now in shovel-ready mode and there are no underground grade separations to complicate matters. This would probably be the second Measure R project -- Foothill Gold Line being the first -- to go into revenue service.
Eastside Gold Line II: EIR/EIS will be released this summer, but it depends on which of the two final alignments Metro chooses to build first (as you know, Antonovich already had the Board approve a motion that whichever isn't the chosen alignment be put on the list for future LRTPs) as to whether or not this can make the deadline. It all depends on how much grade separation is in the LPA.
Green Line to LAX: While Metro is only in "early planning stages" on this, a lot of work was done when the Green Line was originally built and that created a database that can be drawn on for the engineering. That, plus the relatively short length, means that even if the EIR/EIS doesn't happen until 2015 or 2016 it could be operational by decade's end.
Regional Connector: With the EIR/EIS coming this summer, this could begin construction sometime in 2011. Yes, it's probably all going to be underground, and that means tunnel boring and station box construction, so it'll be a tight squeeze to finish by 2020, but it's within the realm of probability.
I-405 Corridor: I expect a Metro Express bus service between Van Nuys and Westwood shortly after the northbound carpool lanes are finished (in fact, Jerard Wright and I are the catalysts on our respective governance councils for this to happen). I don't expect rail in this decade, though; at best, I expect it to be built concurrently with the last phase of the Purple Line, opening at the same time as Wilshire/Westwood Station. But I'm including it here because you could consider the express bus to be a first step toward that Measure R project, and that's very likely in the next couple of years.
Green Line South Bay Extension: A long shot, since only the preliminary environmental studies are being done, but if it ends up largely at-grade, it might be doable by 2020.
Westside Subway: EIR/EIS is scheduled for this September, and tunneling could begin within a year after that. It took seven years for Union Station to MacArthur Park to be completed, and Wilshire/Western to Wilshire/Fairfax is only slightly longer with fewer complications, so I'll say that segment is conceivable by the end of the decade.