In the US, traditional Tiramisu needs to be made at home because it uses raw eggs and it would be too risky for a restaurant or bakery to sell.
For years I have been making the Williams Sonoma recipe. To get the tiramisu to come out firm I had to:
Soak the lady fingers just so. Pour a thin layer of coffee onto a plate. Practice rolling the lady fingers by rolling one in the coffee as fast as you can, another for little slower, and one two seconds on each side. Let them sit for half an hour and cut them in half. The coffee should have made it half way to the core of the biscuit and should not be mushy.
When recipes call for eggs, they mean large eggs, not extra large or jumbo. Larger eggs have more water.
The mascarpone should be at room temperature. Before you mix it in with the other ingredients, whip it in a separate bowl with a whisk.
Add some cream of tartar to the eggs whites. Make sure you whip them to stiff peaks.
If it still comes runny after those adjustments, lower the amount of cream and coffee you add to the dish.
Ever since I learned to bake a Genoise (French sponge) cake, I use it instead of the lady fingers. It took me several attempts to master the Genoise, but it makes a prettier tiramisu.
For flavor variants, the next most popular combination must be limoncello (a popular Italian liqueur) with lemon zest. I have also used left over poached spiced pears in a tiramisu.