I understand that there is a small amount of players that play the expeditions, but the game is too adventure centric, and I think that we, who are playing expeditions should be awarded with some building from it with more then decorative purpose. Not just Expedition castle, but something more, like Harbor for expeditions that could serve as a bonus for colony yield , like those that Settlers 2 had, or arch de triumph, for the valor bonus, and the special storehouse, for storage area of the colony yield for the certain amount of colonies conquered, or level passed.
Also, there's so few specialists for expedition. Most of them suppliers. Only the Black Marshal could be won when the player reach 30 pvp lvl, but that's just pointless. For one to do so, it must gain so much XP PVP, and resources that the Black marshal loose point when gained. Also, lvl30 is the rooftop for the PVP levels.
For the conquering, you could add some more bonuses, or safe time, because some colonies are on high demand, e.g. titanium ore, granite, so you could keep it for only 12 hours before it could be attacked. Maybe some proactive defense options for players who conquered could help this, and also make interesting from the attackers point of view, because this system of pattern defenses are beginning to automate the attackers moves. And defender is limited to what field he could build defense tower. It's not only static defense, but some plots for building just doesn't have any sense.
You could add daily or weekly quest for the expedition, with expedition resources only, and that it wasn't mandatory quest, so those players that don't want to play, could skip these quests. Maybe some excelsior quest/expedition mix will be to much to ask? Maybe all of this is too much to ask, but it's just a suggestion from someone who play expedition a lot, and love that aspect of the game.