I am thinking about creating a 3D game that involves a silly concept. I'm not quite sure if I should implement a serious story driven game or simply make it very re-playable. What do you guys think.

Not enough information at all -- both approaches are valid.

However running with an example:

Your game involves squishing ants. That's all you do - you squish ants - you pick up things to look for ants underneath but at the end of the day it's all about squishing ants. In the background of the game you have small text boxes appear as ants talk to each other -- over time the ants reveal a story about the danger to their kingdom and how they are desperately trying to escape in order to save the kingdom from the danger but to no avail as every escape route gets cut off by the player. Mini-stories develop showing individual ant stories as they attempt to find a safe path. One such story the sending of a small group of youngster/toddler ants on one such journey in the hopes that their size will make them less noticeable to no avail. Over time as the story gets laid out you come to the realisation that you are the terrible danger to the kingdom and that all the suffering that has arisen has been as a result of your unceasing mission of search and destroy.

This is just one way in which you might place a deeper story into a game with little or no complexity or continual repetition.

Hope this helps :)