In my opinion, the diamond should automatically be given to you at 500,000 combined (from all StackExchange sites including StackOverflow) reputation.
The main problem with this is that while reputation reflects, to an extent, your familiarity with the site, the amount of subject matter expertise you have and the level of respect your peers have for you, this is something slightly different than the qualities looked for in moderators. Therefore, on larger sites we hold elections where candidates can present themselves and their vision on moderating the site; on smaller sites the Stack Overflow staff appoints users they know to be capable of being good moderators.
Also, being knowledgeable about one topic (say, solving programming problems) does not mean you're in expert in chess, biology or
life hacks and automatically capable of moderating that site. Combining reputation, as in your proposal, simply doesn't add up (pun intended).
(answers to the initial version of your question)
100k users (actually, 10k users) can already flag deleted posts, asking for undeletion. IMHO, that's already sufficient power. Moderators don't delete posts for nothing; they have been elected or appointed to use that power responsibly.
not even Shog9 (the person with the highest reputation on this site, 343,090 reputation as of 11/27/18) can undelete vote it.
Except that he can, because he is a (the?) Community Manager and has ♦ powers on all sites.
Change the best answer to a question (gives 20 reputation to the user with the best answer)
You mean changing which answer is accepted, right? Not even ♦ moderators have that power, only the author of the question can do that. Actually, it means very little. The best answer, the answer with the highest score, is determined by the thousands of users who form the community. Very democratic, I might add.
Be instantly accepted to work at StackExchange
We simply have too many 100k users for this to be feasible. Stack Overflow alone has over 600 users with more than 100k reputation.