I had a different idea, that might encourage HUGE variety. I dislike typing long posts so here the short version. Sorry for the detour @Jainus
All tropes are assigned a 5 Star rating based on the previous weeks (or 2 weeks) usage/popularity in 20% brackets. This is displayed when choosing your Defense team. Setting your teams using very uncommon (bottom bracket) troops earns BIG bonuses. Using a top 20% troop earns 0. Middle brackets vary accordingly.
Win or loose you get something (Wins are worth more) per battle defended. This is on top of the usual.
Reward will need to be refined to encourage some players to experiment, and allow others to run their usual grief. Variety is the goal.