Preserving their ability to be transferred is always a plus, if possible. I'm also in this boat and just opened a Business Rewards Gold with the annual fee waived to keep mine in the variable state when I cancel my plat after 1/1 and the new airline credit.
That being said, if you do need to transfer, British Airways Avios is a popular transfer partner. They have a good bonus on currently (40% I think) and can be used for many good One World flights. BA flights will have more taxes if in London, and Fuel Surcharges, but partners often are great options, even overseas besides AA flights in the US. I haven't read your blog to see where you travel, but I use Avios in South America (LAN) and Asia (CX and MH), and have planned some trips for a friend with Air Berlin in Europe.
Singapore, ANA, and Air France Flying Blue have some good values if you look into their programs.