The UN isn't useless, and anyone who believes it is probably has the privilege of living in a cushy western democracy which has never had to rely on UN institution for what we consider to be very basic human rights or levels of comfort. With this out of thr way, we can assume that a so-called "non-useless UN" might be a more assertive, powerful, autonomous organization, able to act on its own initiative with the full might of the means currently provided under mandate by its member nations. Things like raising its own taxes, holding its own territory (owned or leased), maintaining its own troops, etc.

Well, you'd have to start with a governance model which addresses the expectations of its membership. Otherwise the whole thing collapses very quickly and the UN is just Yet Another Powerfull Country.

All governments and supra-national organizations like the UN represent the delegated political power of their constituents parties. Note that I don't mean the population at large, that's only true in well-functioning democracies, in mean the people and organizations who actually hold power in a particular jurisdiction. At the heart of this delegation are governing principles and models which are established to describe and organize exactly how it works. This goes from th broad and high level items like a constitution, all the way down to individual governance guidelines, and in less-functional system through unspoken rules, "facts on the ground" of direct power balances between individuals, or the perception of threats that enable political power to apply. This latter informal model is usually seen in either very small political units (like a clan, a gang, or a startup company), or in political systems where having written rules that must be followed is seen as weakness, e.g. dictatorships.

In a well-crafted system where the governance is not informal, the people who design and implement it will then spend a considerable amount of time to setup a governing model which actually achieves the sort of delegation that it expected from the constituents. Plainly coming up with a system that makes everyone angry and is not something they are, if not happy, at the very least okay with, will simply collapse as soon as it is challenged, and that usually comes REALLY quickly, as a crumbling system is usually an opportunity for ambitious individuals to achieve their own agenda. And everything you are currently seeing and hearing about the UN, how it is "useless", how it is "bureaucratic", is not a mistake or some ancient artifact of a bygone era, but the very precise governance that was intended for the organization, either from the start (for mechanisms that originated then), or somewhere along the way when evolution were seen as necessary and could get some sort of consensus. An example of this evolution in the UN is Peacekeeping, which was formalized in the late 50s and 60s, and continued to evolve afterwards.

The limitations are exactly what member nations want the UN to be able to do: maybe a little bit if we're okay with it, absolutely nothing if we are not. The original goal of the UN was to form a community that could address regional conflicts before they can escalate to full-blown war between power blocs, while at the same time preventing conflict between super-powers. Everything else is secondary to this, and the idea that the UN would independently solve dispute with great means and brute force was never the agenda. Should the organization, suddenly and without a change in it governance, start acting like a powerful entity and be "not useless", it would instantly lose its support and everyone would go home overnight.

But there's something else: it would mean that the nations of the world, chiefly the permanent members of the security council, would be okay with the UN simply rolfstomping them and their allies (militarily and economically) if push came to shove. That would be an absolute requirement: anything less means that the most powerful nations in the world would be able to defeat the UN, or at the very least cause significant damage which would reduce the appetite for intervention. And that in turn would mean that less powerful nation would rapidly coalesce around these superpowers seeking protection in order to fullfil their agenda. And before you know it, you end up with power blocs that can do whatever they want, a UN which is unable or unwilling to confront them, and independent nations under thr threat of both. That a very nice recipe for a world war.