You'll probably finish less than 5% of the missions/quests when you hit level 20. There are going to be a toonnn more missions and quests for you to do after you hit 20, the bulk of the storyarc content is all level 20, and theres even more other stuff.
work for titles, there are various titles you can achieve by doing different things in game. this ranges from capturing all the game skills to drinking so much that you earn a title of 'drunkard'.
elite skill capturing... Theres over a hundred elite skills to capture, and the more you capture the more versitile your character will be. Learn all the secondary professions for your character, and get some of that professions elite skills to help you make new powerful unique builds
item hunting... Although items arent necessarily better than eachother, some look a lot cooler, some are known to be harder to get, and its important if you want your character to be versitile that you have a wide range of weapons at your disposal
Challenge missions, factions and nightfall have a bunch of challenge missions that you can complete over and over. Completion is a matter of degree, not kind, which means some people can beat the missions better than others. A record is shown of the people who beat the mission the fastest, and you can be legend if you make a group superior to others and take over the records logs.
Running/map exploration/helping newbies/learning the games lore, go back to old places and help other people or explore areas you missed, learn to run past enemies and help other players by running them past hard areas, talk to NPCs you missed and learn the lore behind the game
get involved outside the game, contribute to a fansite or to guildwiki, record videos and post them on youtube, post on forums to discuss strategy, do all that other fun stuff outside of the game!
armor hunting... There are a lot of different armors, some much more expensive than others... The best armors show your seniority in the game
become a merchant, buy for cheap, sell for lots, there are a lot of people who want to get rid of their crap or dont know how much thier items are worth, buy their stuff for really cheap, then sell other stuff to them for very high prices. If you end up with two much gold, buy koss some 15k armor, give the gold to a newbie, or find some other revenues to get rid of the gold
Make your heroes as good as you, because youre going to have like 10 of them per character, and they all need weapons and armor and pets! Hehe, I went and got a black moa bird for all my heroes, and now when I do missions with 3 heroes I make all of us secondary ranger and bring my black moa squad, very fun.
Make a guild Help your guildies rock as much as you
And if you did all that stuff, then you might want to think of taking a shower, because you have probably been playing since prophecies release without moving from your chair, and you smell