My group loves to eperiment with new (and old systems)...
If you wnat to keep it a D20 game, then something like Star Wars or CoC or even DragonStar would be quite a change of pace, and some of the D20 games in Polyhedron would be great..The Pulp game should be a good one (the name escapes me at the moment)...Pulp games are always alot of fun...
But we really like to try new and different systems, so I run failry short campaigns with lots of action (though not neccessarily alot of combat)....D20 is cool, but there are ALOT of good games out there...learning a new system can be a pain, but at the same time it's a good energizer and can really get everyone excited, since everything is new and fresh.
We were initially very dissapointed with 3E, so we tried a few of quick one-off games with different systems 'till we decided on Exalted....it was our first experience with a WW game, and I downplayed the Anime feel, and ran it like a Runequest/Glorantha game "on steroids"...we had a blast...
After that wound up..we wanted something different than Fantasy as well...So after some discussion and myself purchasing some different systems we went w/ a West End Games D6 Star Wars game set in the prequel era...We have about 2-3 more sessions before it's wrapped up..my players have since expressed an interest in trying another 3E game, but we are also looking hard at Earthdawn, and Lejendary Adventures...
Sometimes a unique fantasy world or game such as Earthdawn, Exalted, Tekumel or Glorantha is more than enough to get people charged up again...My players are unfamiiliar with Earthdawn (and I only know the basics from when I owned the book a few years back), but the setting is very intriguing to them as it's both familiar and fresh at the same time...In fact they were pretty much sold on LA till I explained my interest in buying some Earthdawn materials, and gave them a bit of a primer..
Although you should dicuss and all come to some sort of an agreement, dragging on through a game you are bored with is no fun, and should be avoided.
Why play if it isn't fun?