A projector would certainly be a fun idea if you don't have one already and want to get a theatrical experience going.
For the sound portion, your receiver is really the strong point in that system since it's a $200-$300 receiver new which is a good one to keep.
Speakers or sub is going to make the biggest difference.
If you listen to a lot of music, the front stereo pair is going to be where to concentrate. If you aren't already, try listening to music in Stereo rather than a surround mode. With just two speakers and the sub going you're getting playback as was intended.
If you watch mostly movies, the front three speakers (especially the center channel) are the most important. Matching the front three is a good idea, so if you do a decent amount of movie watching buy want to do an upgrade for music (stereo pair), it would be a good idea to upgrade the center to match at the same time.
The surrounds aren't a big deal. You can even move the fronts to be surrounds if you replace the fronts.
Unless you listen to DVD-A or some other kind of multichannel music, the surrounds aren't going to be used much and it's not a huge deal if they don't match.
If it's a choice between upgrading all 7 speakers to a medium upgrade or upgrading the front three to a good upgrade, do the front three.
Ok, there's also the sub.
How big is your room? How satisfied are you with the amount of bass the Onkyo sub puts out? Any considerations like living in an apartment etc?
So, lastly the question that probably should have come first: What's your budget?