Soulstone building (Altar of Souls)
Considering how much soulstones are needed to train SH units and how hard it is to get enough, except if you rank in the PvP and other tourneys every time, I have a suggestion. Actually it originates from a league-mate whom I totally agree with.
Add a new building. Call it Altar of Souls - or something like that. Based on Altar of Mara, it allows castles to sacrifice resources (even extra Adamantine) for Soulstones.
That solves two issues in one. Excess Adamantine that everyone has an issue with, and the lack of soulstones. Instant balance created.
To make it further interesting, allow sacrificing just ONE type of resource, out of the 4: gold, silver, food, adamantine. This way, players can get rid of just what they have excess of. As this is mostly in players' favour, set the sacrifice a tad higher. Balance again.
Plarium gets to sell another upgradable building to every player, i.e. revenue, and players get something they really need. Did I mention balance?