Well since its lego anything really, here is two gameplay ideas: Morphing units.
1) Since fun of lego is building stuff, then adding more stuff to it. I think units should reflect this, by having morphing tiers. You build a tier 1 castle then you can upgrade it to tier 2 then tier 3 and so on, which each castle getting larger with more bricks, towers etc.
2) You could make a sort of morph tree to give a player some customize-ability. For example you could produce a basic unit lets say a robot. Simple small unit with a weak weapon. Using a morph skill player can upgrade unit, however given 2 or more choices to what to upgrade to. Lets say a Melee unit Ranged unit and Anti Air. Then the newly upgraded tier two unit can have a 2-3 morph options and so fourth, so a melee unit can morph into: fast melee or armor melee. A range unit can morph into: speed range or power range. And the Anti air can morph into heavy Anti air or a long range anti air. With each upgrade getting more bricks, accessories etc. Getting stronger as a unit.
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The second choice will be a lot of work to warn you so it just a suggestion
As for a theme I'm not sure, but since its lego you can get a way mixing things like tanks and castles, dragons+ spaceships etc just fine. Its lego no need for a realistic world. So just think what would be fun to include.
PS Sorry about that Eliminator, that is a good Skateboarder model