The "Pick Nth Instance" system condition works as described, picking only out of currently picked instances. So we kinda do already have it.
The object(n) expression does not seem to care about picking, but can reference the full instance list of that object.
The suggestion is about referencing UID in an expression. But its probably more of a quality of life niche case type of thing, as picking by UID can already be done by condition as well. Might save a few events here or there. The example Tacker gave could simply be done with a For Each player instead, or add an instance variable to narrow the scope by picking as normal.
The ability to specify only picked IID's (they call them SOL ID?) in expressions might facilitate the whole "how to I refer to and manipulate two instances of the same object colliding or otherwise interacting" without resorting to the common object and family workaround though! Maybe. That would be the fireball part of Tacker's example, and that's where I would be really interested. But that is what Ashley dismissed as already existing, which doesn't seem to be the case, so it's probably good to call attention to it.