I've had this idea for a while now:
From Wiki:
"Vikings were the seafairing Norse people from southern Scandinavia, who from the late 8th to late 11th centuries raided, pirated, traided, and settled throughout parts of Europe."
As a game, you have the raided and pirating part down, plenty of attacking and looting going on in this game. Epic battles definitely abound. You have the settling part down, joining clans and forming tribal bonds. What this game entirely lacks is the trade aspect of Viking culture, and there is no shortage of items that could be traided around.
Gold nuggets for example. Once your town is upgraded fully, and you've gotten the planner achievement maxed out, there is no use for gold nuggets anymore, but you still earn them in contests sometimes. Same with Obsidian. Once your T7 tech tree is maxed out, there is no need for them any more, but you still earn them mining level 8 and 9 mines. Other items could include invader materials, you only need some for making and upgrading your hero gear while others build up that someone else might be willing to trade for. Gems, hero keys, rune fragments, bronze, manuscripts, totems, warriors drafts, soul shards, star shards, tons of different types of scrolls from resource scrolls to upgrading scrolls to fighting scrolls. In other words, pretty much any item aside from basic resources that we can already send to each other in convoys.
Items such as these and more, could be bought and sold for gold, or just traded across on terms that any 2 parties can agree on. I'd recommend a laissez faire approach from Plarium, where instead of trying to make set prices or trade value for these items, you allow each individual Viking decide on what they'd be willing to trade and/or sell items for, and any transactions be agreed upon by them.
I imagine my suggestion taking form not entirely unlike Holmgang or Jotunheim. A seperate place, perhaps islands like Holmgang, or a whole other map like Jotunheim, but a different place Vikings can journy to for the purpose of barter and/or commerce of in game items with each other. It could always be open like one can always journey to Helheim, or it can occur occassionally like Holmgang or Jotunheim. The purpose of this place wouldn't necessarily be fighting, but I suppose there's no reason why fighting couldn't happen there too.
What's in it for Plarium? Well, you could do like what you do with convoys, and extract a tax rate on transactions involving gold, or perhaps make each transaction cost a small set fee of gold, or perhaps you could make the pass to get to this place an item(like great relocation scrolls, journy to Holmgang, or guide to Jotunheim). They could be earned by passing so many checkpoints in certain contests insuring continued game play to use this land of marketing. You could sell passes like drakaars in bank offers, though I'd hate to see that be the only way one can get there. You could make passes available in the clan store. There are any number of ways Plarium could benefit along with us Viking users from this idea.
We do not need any more big fighting in game contests. There is already plenty of fortress fighting; POP, SH's, Towers of Fury, Jotunheim, and Holmgang, as well as plenty of town and tile fighting. We do not need more fighting upgrades. The amount of upgrades one has to do to become a powerful fighter is already overwhelming and discouraging to newer players. What we do need, especially if you'd like to reel those newer players in and keep them interested, is more ways to gain these valuable in game items that help them grow to become powerful fighters.
I really think an upgrade such as this one would add a lot to the Vikings community and gaming experience.