Why would we attack hero party? I'm confused by this line of reasoning, we had a cause before, now we don't...
Because if your faction was looking to expand its territory, they would be the most promising target of the factions whose domains are bordering yours at the current time.
You have a two month cease-fire with Norse Horde, so they're not an option unless you want to ensure that no one ever makes any kind of treaty with your faction again. The Remnants' fiefs in the tundra would be easy to attack but difficult to hold because of their positioning and also because they're right next to the lands of the Free Companies. Astralis is friendly with your faction, and you would lose more by trying to grab their lands than you would by establishing agreements that work towards your common goals.
What's left is hero_party. They have fiefs that are highly developed, in close proximity to the region your faction controls, and probably not as well defended as they should be, especially in light of the fact that they are currently dealing with BIRD CLAN. That should be all the cause you need to attack them, even before you consider the fact that they caused problems for you earlier in this round of Strategus.
As for whether it's actually a good time for the Hospitallers to consider expanding or not, that's rather obvious too, considering you guys have over 11,000 well-armed troops at your disposal in the steppes and are no longer at war with VE and Norse Horde. If you left behind at least 4,000 of them to defend against FCC and other possible aggressors at key defensive points, designated 1,000 of them as a standby force, and took 6,000 of them to invade hero_party, you would, as long as the diplomatic and logistical side of things was handled correctly, have a pretty good chance of winning a major victory without putting your heartland in jeopardy.
In short, you have pretty much every reason to go for the gold, which is why I expect you will at some point. This is assuming your faction is still as ambitious as it was and is not so worried about the threat posed by the Free Companies (and possibly the Remnants) that you are just going to hole up in steppes and remain completely on the defensive for the foreseeable future.