The UN Security Council could be more democratic, effective and legitimate if all nations are treated equally. Give all nations equal chance to be elected. Restrict the number of allies that can serve on the council at the same time to restrict voting blocs and broaden the represented views.
Get rid of the veto power for any one nation, as this has rarely been used for good and is usually misused to protect something bad. Give a time penalty to the countries that have vetoed.
Too many countries have territory disputes and they lack the motivation to resolve them fairly. There are many examples of countries with power taking territory from neighbors with less power. Find a way to resolve these disputes before serving on an important and influential body like the UN Security Council.