A few fairly specific items, which I plan to tackle in the relatively near future:
1. Play a bunch of links golf in Ireland & Northern Ireland - my first links round ever was at Portmarnock, but haven't played golf on Irish soil since - by far the biggest hole in my experience
2. Play in the Scottish Highlands - I have visited Scotland several times, covered most regions (including both big names and lesser-known courses), but never made it North of Cruden Bay
3. Play the "best of the best" in the US - I have played roughly half of any top 100 US list, but not many of the top 10-20 - beyond just wanting to see these great courses, I'm genuinely curious whether there really is a significant gap in quality.
4. Attend the Open Championship - I've been to the Masters, US Open, and PGA several times each, and figure I ought to get to at least one Open. While it is my favorite of the majors, I have opted to spend my limited time in GB&I playing links golf and not watching it.
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A couple more that I have in mind, but it may take a while before I can really consider them:
5. Play the top 100 links courses, play every links course - I am not a list chaser, because pretty much every list has at least a few courses that I'm not terribly interested in visiting - BUT - any list of links courses, particularly those best regarded, would seem a fun one to give a shot.
6. Play in Australia & New Zealand - I imagine it will probably take multiple trips (of 2+ weeks at a time) to see what I would like (golf and non-golf) - cost and time limitations put this out several years for me.