Yeah not sure if there is enough demand to put this in core, but fairly simple to do in a plugin. Create a new route like @julian suggested and then rewrite the url to the latest topic.
routeHelpers.setupPageRoute(router, '/latest', async (req, res, next) => {
const db = require.main.require('./src/database');
const tid = await db.getSortedSetRevRange('topics:tid', 0, 0);
req.url = res.locals.isAPI ? `/api/topic/${tid}` : `/topic/${tid}`;
next();
});
Then you can set your homepage to latest and it will show the latest topic as homepage.
