I think you should walk before you attempt to run and the two events cater to some of the same people, but some a different audience too, so it'd be cool to do both. I'd be keen to attend such a campaign too on another occasion (I just didn't want it to replace the tournament), but it would involve a
lot more work to organise and to get right. Do this tournament really well and people will be more likely to come to something like this.
To keep it themed rather than coming up with silly background ret-cons for them being there (e.g. the Tau in the Medusa campaign) you should require people to use any variety of Ork, Chaos or Imperial armies. I'd be surprised if most people didn't have at least one of those armies (with spare armies they could probably lend to people who wanted to play but didn't have the correct sort). Be open from the start what it's all about and check what armies people want to use when they register (even restrict late-coming people to play on one side or another if the other side's spaces are full up to keep it equal). Sounds good to me

Could also be good publicity for epic in a way if you could get some photos of some
really big cool battles and bung them on Warseer and the like.
edited to add - re: scenarios you might like to talk to Chroma, as he's in the process of producing his
Total War fan supplement with loads of scenarios and the like to add more variety to epic play.