The possibilities for tampering are endless! - Here are some suggestions:

Hops: Add optional hops for extra aroma
Sourcing: Use Philly Sour yeast
Low carb: Add Amigase for full fermentation
Gluten reduced: Add Brewer's Clarex
Increased strength: Omit the water, or add extra fermentable sugar (malt extract etc.)
Lactose: Give the beer an extra smooth body and sweetness
Other additions: Oak, coffee, spices, orange peel, fruit/puree, cocoa nibs...

Suggestions for hops and yeast:

As mentioned, we want to encourage experimentation. Nevertheless, it can be good and reassuring to have some concrete suggestions, which can also inspire you to make your own adjustments. Here are some suggestions:

• For example, use one of these yeast types : Lalbrew London, Empire Ale M15 or Lalbrew Windsor British-Style.
• Try adding oak cubes during secondary fermentation for a fantastic oak barrel flavor to your beer. 
• Using lactose, cocoa nibs, tonka beans and or purée can produce a great Pastry Stout !
• If you add  dry hops, for example 15-20g Amarillo and Centennial on day 7, you get a Tropical Stout.  
• Try steeping good, coarsely ground coffee in the fermentation vessel a couple of days before bottling. You'll get a magical Coffee Stout !
• Get a Baltic Porter by adding lager yeast, such as Lalbrew NovaLager or Californian Lager M54 

FWK Creative also works great for using up leftover hops!