My guess is computer hardware and a source of electricity, so he can finally get started on one of those big projects to gain large amounts of personal power / resources.
The economy-disrupting arbitrage scheme was introduced as early as chapter 4. Lots of money would have helped Harry in pretty much every arc. He could think of many, many other schemes to help him finally start world optimisation in earnest. But he never did that. In story logic, he never got round to doing it due to interference from all sides. In writing logic, this can only happen in the final arc because otherwise, the story will be known as "Harry Potter gets really rich" and cease to be about the methods of rationality anymore.
That doesn't necessarily point to computer hardware, but chapter 87 mentions them as a powerful tool that wizards lack ("If there's a money-making method in this book that sounds difficult for a wizard, but it's easy if we can use Dad's old Mac Plus, then we'd have a plan.") and I have a hard time thinking about world optimisation plans that wouldn't benefit from at least some sensible organisation of data, scheduling and on-the-fly printing.
This would have the inconvenience of requiring an undefended secondary base, perhaps in Hogsmeade or London - Muggle technology doesn't work in Hogwarts, and Harry can't Apparate, so he would have to keep travelling back and forth to wherever he kept his computer whenever he wanted to use it.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 98. The previous thread is at nearly 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system.
Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: