There has been some discussion about enhancing the router API to give us a lot more flexibility overall. But that will probably not happen within a reasonable timeframe, so let's move to discussion of a stop-gap that works but maybe isn't as nice.
We could perhaps have an extension point like "jenkins.main.routes.default.redirect" which requires a single <IndexRedirect /> element to be supplied. We could provide a default that which points to "/pipelines" but let a custom one be supplied to point elsewhere.
If multiple plugins try to contribute we need to determine who "wins". Keith Zantow has used an "ordinal" property on extensions to allow them to be sorted, and we could use that to pick the right one from an array of default redirects.
I don't expect this to be too difficult, could whip up a PR tomorrow probably if that works for you Thorsten Scherler ?