Speaking from a graphic designer's perspective…
Whatever you DO use, you have to make sure that the images and text are clear and legibile. Even if you think a certain scene is particularly "awesome", it doesnt mean it will lend well to a t-shirt design. Keep in mind the difference between a design for t-shirt and just slapping a panel from the manga on it (something that is done a lot and pretty lame imo). Concerning any art you use, especially characters... you have to think about how you'll crop the images and I recommend tracing them in illustrator. The amount of shading and detail that Oda puts in would not lend well to a shirt, and of course resolution is an issue (for t-shirt designs a vector format is always better that a raster which pixelates when blown up).
I think some good ideas have been to imitate some of the t-shirt designs that have been featured in the manga (like some of Nami's or w/e). Incorporating "logo-like" designs would also mean less work for you (like the Galley-la logo, or the Straw Hat flag), but the question is how interesting would they be? Oda does some pretty interesting stuff with text and typography, especially in his colour spreads, that would be cool too. For example the "excuse me, no parking on the battlefield baby" phrase. Stuff like that.
If you do incorporate scenes or characters, pay attention to what elements of the scene you incorporate and what to leave out, and also how it is framed and cropped. One scene I can think of that would be cool is... at the end of the Arabasta arc, when they say goodbye to Vivi... they lift up their arms and you can see the "x" on all their wrists.
Anyway, just some things to consider I guess.