To elaborate on JK's notion that prominent peaks are somewhat overlooked:
I think people are missing out by going after elevation thresholds instead of prominence thresholds. While I haven't come across a category of peak that I couldn't motivate for, IMO high prominence peaks are overall the most worthwhile. A prominent peak is more likely to both have a view and, as the highest point in the area, feel like a real summit rather than just a bump on the ridge. I've found it's easier to get "civilians" motivated for major prominences than for random P300 peaks. If you like visiting new places, prominent peaks are far more widely distributed than elevation threshold peaks.
Some good lists to go after:
In Colorado, once you've finished the Centennials, the P2K peaks, are a sensible next objective. It's an easier list than the Bicentennials but more diverse, it will take you to a lot of new places. There are some long hikes, some near drive-ups and some scrambling but nothing that requires a rope. As far as access issues, only a couple of the peaks are modestly problematic.
The "penultimate last great problem" of Colorado peak bagging is the complete set of 425 P1Ks. Mike Garrett seems to have run out of steam heartbreakingly close with only 9 remaining. Many of these peaks have access problems and there is at least
one real show stopper. For my money, the single best peak list to go after in the contiguous US is the one comprising the 57 peaks with at least 5000 feet of prominence. This list has it all: drive-ups (Pikes, Mount Washington), multi-day approaches (Gannett), easy hikes, snow and ice (Cascade volcanoes), 5th class (Grand Teton) and all sorts of random objective difficulties such as possible encounters with caravans of people crossing the border surreptitiously (Mt Miller), grizzly bear nesting grounds (MacDonald) and closures that rule out the entirety of the good climbing season (ibid). These peaks are in 13 states and two of them are east of the Mississippi so you'd see a lot of the country doing them. To John's point about the relative neglect of prominences, only eight LOJers have completed the 57 Ultras as opposed to the 35+ who have completed the 638 Colorado ranked peaks >= 13000'.