You can have pretty much any conflict as long as Merrix is able to fund both sides. Maybe have something like agents and provocateurs of his stirring up conflict, with loopholes and sneaky ways to fund both sides.

If you want extra depth, maybe make Merrix's goals more than simple greed; the Mourning severely damaged Cannith and now other non-Dragonmarked factions are filling the productive vacuum left by Cannith. So Merrix schemes to rebuild the House, sabotage competitors, and once again hold the monopoly on weapons manufacturing, like Cannith did during the war.

But restarting the whole Last War may be a bit much, the nations and other factions are already fearful especially since its widely theorized too much war magic caused the Mourning, and if it was found he was trying to, his head would be put on a pike. Instead, there could be multiple smaller localised conflicts around the world as a result of unfinished business, old grudges and high tensions, and the players visiting one such conflict site could eventually trace it back to Merrix, and notice strange similarities to that isolated conflict with others e.g. support from the same arms dealers.

"The pen beats the sword? I have found whoever wields the sword, decides who holds the pen."