For this idea consider either honor or a new currency. Or possibly keys—I heard it used to have a functional role.
You use the currency to buy and upgrade capital/permanent things (eg traps, minions, pets, mini-bosses). You can buy new floors. Possibly 1x hero can be used per floor as a mini boss and possibly the final fight is against a 5-man hero party.
Same as regular tower except you choose which tower to engage against. Each floor will grant you x amount of currency but raiding the final tower will steal currency from your opponent.
I’m thinking sort of like how Clash of Clans allows you to build a base... you build your tower.
This is just an idea, but the profitability would be in people paying gems to upgrade capital items—to shorten the time.
Currency is used to buy and upgrade tower items. But it can be used to buy rare promotional items as well.