It would be helpful if we could have a redstone signal that increments/decrements a counter.

I think this can kinda be done already with a proxy and materializer, with pipes/tunnels moving variable cards between them. Proxy will hold the current value in it, while the materializer can have a "proxy + 1" card in it. Materialize a card and then insert it into the proxy, and you can increment. But it's a pain.

In fact, it would be even more awesome if it could apply an Operator to the value inside on redstone pulse, mutating it to whatever the Operator's result is. Then it could be used for things other than numbers. You could put a list and a Tail operator into it, and each redstone trigger would lop off the head of the list.

Or, at least it would be that simple if the materializer wouldn't jam up as soon as its write ouput slot couldnt be emptied before its write input slot was filled. Sigh.

With the Integrated Tunnels addon, you can pipe two variable cards between a materializer and proxy. Importer and exporter on the materializer, interface on the proxy. (Which is a similar setup to what you suggested)