We had another reserve currency before the USD, it was the gold standard - and we moved away from that for a reason.
There is possibly an alternative that I can think of for a global currency, and that would be a currency that is composed of a basket of currencies (sort of an index currency) based on some formula. I fear creating a global currency with a strong organization like a UN Fed - I just don't think the UN is competent enough in other areas.
For example:
If there were three participating currencies (to keep it simple), and based on that, the US GDP was 50%, China 15%, and EU 35%, then the currency would be composed of 50% USD, 15% Chinese Yuan, 35% Euro's. The value of that currency would then be based on that. If one currency devalued considerably, the would be held up by the other participating economies. This would allow the Chinese to demand the US issue the debt in the the World currency - which could be less risky than having it all in USD.
Of course the transition would likely create severe financial shocks which would cause a major depressionary event - which should be avoided if the US does not royally mess up their own currency.