Build-Your-Own Stats
 

I'm the type of person who loves analytics, and from time to time I try to create my own statistics. For instance, I can't stand judging a reliever by their ERA, as many relievers can blow up and give up a ton of runs, but most of the time are solid. This skews the ERA data and makes it so ERA doesn't show me if a reliever is doing their job. I feel like a reliever is doing their job when they go out and give up no runs, so I use a stat sort of like saves/save % where it shows how many of their outings they give up runs.
Example: Dellin Betances had 66 relief appearances last year, through which he had an ERA of 2.70. However, he held the opposing team to 0 earned runs 55 times, and allowed 1 or fewer runs 61 times. I feel like ERA and other stats may not show how well he did his job (which is to give up no runs when he pitches), so I instead look at this stat I made up, which I'll call a Perfect Relief Appearence, or PRA. Betances had 55 PRAs, or a 83.3 PRA%. This stat may seem similar to holds, but with a hold you can allow up to 4 runs in some situations and still get a hold. PRA shows me how reliable a relief pitcher is over each outing, which is something I value a lot.

This was a very long winded way of getting to this, but I would love if there was some way to add a build your own stat sort of mechanic in OOTP. It's a pain to calculate stats like PRA by hand, and I don't really want to do it across entire leagues. I understand that there would be so many options for making every type of stat possible, but I would greatly appreciate something along these lines.

Funny thing...I was going to suggest the same thing, and for much the same reason. Somebody in an article about two years ago, and I can't remember who, proposed "Goose Eggs" as a stat for relievers - roughly, it was how many times they pitched an inning and allowed no runs to score, which is really what you ask of a reliever, anyway.

I'm not sure how "integrated" a custom stat could be with the game, but perhaps it could be an addition to the sortable stats page...maybe present a table of variables and allow a formula to be made? Not sure if that would do the trick exactly, but it'd certainly be interesting.