The basic FiveCore mechanic is pretty simple and very popular: Roll and if it's a 1 or 6, something happens.
Its easy to adapt and you can find a solution to an unexpected situation pretty easy.
What I came up with is to make a supplement of essentially pre-configured rolls for a bunch of different actions. Anything from persuasion and detective work to climbing up buildings and walking on narrow surfaces.
For each action, it'll give a few comments on how to handle it and then the results if you roll a 1 or 6 attempting that action.
The idea is to give players a whole bunch of new options to use in scenarios or even on a regular basis.
This can also form a good basis for future projects. For example, rules for travel and scrounging can be lifted into the future post-apocalypse expansion easy.
An example I just finished:
Acts of strength
This can be used for most actions requiring physical strength and brawn, including pushing, lifting and breaking things.
As with acts of dexterity, this can be used in place of other rolls, for players who prefer to use a small number of tables.
The attempted action is assumed to succeed unless a result is rolled on the die.
1 Don't have it.
Character can't do it though another may attempt the same action. Two characters that both failed can join up to get another attempt.
The character fails as above and is in intense pain. -1 to movement rates, rushing and brawling for the rest of the battle.