Although I live in Japan, I'm from America so I'll write about America.
If I were US president the most important thing would be term limits for the legislative and judicial branches.
Today's career senators and representatives care only about re-election. That's it. So their lives are crazy, endless, bottomless buffets. They become detached from their constituents. They work for the lobbyists, not the people. They put their own futures ahead of the nation's.
Judges in the Supreme Court and federal courts serve for life (or retirement). That's a long time. Although they don't have to worry about re-election, they easily get detached from the people. It's the same problem as the career politicians.
So what's the limit? I don't know but I think twenty years is enough. For a senator, that's 3 terms (a senator serves 6 years per term). For a house representative that's 10 terms (a rep serves 2 years per term).
Put a clear cap on the years of a politician and he/she'll be in a hurry to get things done for his/her people--especially towards the end. If policies work out, great. If they suck, re-do them. It will be brutal and ferocious but democracy is like that. They will actually show up for work!
There are other things of course but term limits is the most important for me. Give me that and it'll all be worth it.