From my experience most of zileans utility is already in his kit and his tradition builds dont improve it much or are just overkill when is comes to cooldown reduction and mana regeneration. By having more damage and using you mana and abilities more wisely you can have almost the same utility while also providing a big chunk of ranged damage to your team.

As a support it is really important to have an adc that benefits from your kit and can either draw focus for easy fights in the early game or can run the enemys down with you together in the late game. |
As a toplaner there are a lot of hard matchups as well so try to not feed and if that is the case always remember to support you team later on or you will be just useless.

Depending on your adc either engange hard in pre six or play passive to let your adc carry in the late game. Because of you damage you are also the monster for 1v1's when you have 2 items and your slow plus dmg allows for a lot of ganks that help not only your team but also lets your scaling adc get solo xp.

Toplane is really matchup dependent. Most of the time you shouldn't have picked zilean since there are much bette roptions for top but you can be effective in: 1v1s midgame and beyond (with that and your great escape) split pushing and supporting a hard carry with levels and your utility.