It would be neat to be able to display a more widely used review score on games, including on shelves (and for sorting them). Steam scores or aggregations of various other web sites’ scores could be interesting, but I suppose the ideal would just be to show metascores when they are available.
Metacritic does not appear to publish a public API, and scraping is prohibited by their terms and probably also via technical means as well. But the scores for many games are available on Wikipedia, which it seems is readily available via public APIs.
For example, you can even see the metascore for a game via Wikimedia REST API using the command line, without any code at all:
$ curl -s https://en.wikipedia.org/w/rest.php/v1/page/Einhänder | sed 's/.*MC = \([0-9][0-9]\) out of.*/\1\n/'
89
This is obviously not how you would parse a Wikipedia page for real, and the format varies slightly by platform (and often there is a score for each, which could just be averaged), but the point here is just that the data is often publicly available.
Then, an interesting question might be: when would be a good time to fetch this data? One idea to limit volume is to simply do it on page load when it appears on game page or shelf and then cache it for a long time.