People all around Warhammer are pushing for some sort of expansion, asap. New content, fresh things to do, something to bring people back. Start on it now, release it last week.
Is the timing right though? From a business perspective I started thinking about it and I am not so sure. This post will of course be filled with assumptions and theories.
Assuming an expansion is in the works, first off, there is information out there suggesting that no box expansion can be released before the middle of next year. That does not mean there is even one in the works, it is just a note. More likely is a free expansion similar to TOVL, which could come out at any time. If a free expansion was close to being released, I am sure Mythic and/or Bioware would already be hyping it for a fall release. This would put it just before the release of the new Star Wars MMO.
If I was Mythic Bioware, I would make the assumption that regardless of what you release before SWTOR, your going to have a large number of people at least try SWTOR. Meaning if you release an expansion this fall/winter, you will bring some people back, and then take a large dip in subscribers when SWTOR comes out.
If you plan for something ~3-6 months after SWTOR then you would, assumably, have a solid update on a very solid game to draw people to that are not enjoying SWTOR. GW is also scheduled for release early on as well, so if you come out just past both of those, you can possibly grab a lot of players and in a strong marketting campaign that almost acts as a re-release of WAR, it could do very well.
Obviously there is a pretty big risk waiting almost a year to release a large expansion. The biggest risk being keeping minimal subs and profits on a game that many would argue is dying fast due to the lack of growth in the content section. I can only imagine this must be somewhat difficult for Mythic, who are still pouring their heart and souls into this game, but in the broad company scheme, the rate in which it is shrinking and with Bioware releasing its MMO soon, Warhammer is quickly becoming unimportant.