well, my only advice (I don't play ranked because of having a terrible connection and lack of appropriate runes) would be to play as though you are playing with the elo tier you are in, this way, your team can appropriately adapt
another suggestion would be to use champs that have a lot of versatility, like jinx with two forms of crowd control and a global ult, reksai can control the entire map, etc there are plenty of team-friendly champs
the hardest part is getting people to agree on roles. I play ADC, mage and support, but if those three are gone, that only leaves top and jungler if the team wants perfect synergy. gangplank I enjoy top, so that would leave one position unfilled (I consider gangplank an ADC top because of critplank )