Substitution

The green peppercorns in brine will give both some heat and some saltiness (from the brine).
To add these tastes using other ingredients, possibilities include:
more capers (but not the full 2 tbsp unless you love them)
dried cayenne, black or green pepper, 1-2 tsp
some chopped pickle - about the same volume
some chopped fresh chile - 1 small red