There isn't one. You've got the US or nobody at this point, unfortunately.

Your belief in US exceptionalism is interesting. Even more so, because it is so obvious that the power that will soon overtake the US economically is very likely to fill that void. But they will also get all the benefits of that role and the US will slowly lose them.

In fact, they have already been stepping in the US' former shoes. Some EU leaders mentioned to Zelensky that the only reason that Russia hasn't used nukes against Ukraine is because China said 'no'. In better (depending on perspective) times, it would have been the US as its role of world leader/police to enforce such implicit rules.

It is true that China is currently supporting Russia, but it's mostly of their interest to drain NATO and increase their power over Eurasia. Well, thanks to Trump they proceeding very fast. Or as the Chinese like to say "Trump the nation builder".