One strategy for Yahoo: Turn Google's strengths into weaknesses by opening up all of Yahoo's products (Search, Mail, Finance, Maps, Sports, News) to developers to be maximally programmable. Allow developers to monetize in any way they feel like (aside from fraud) and let them link together apps in creative mashups. This links together YQL, Yahoo Pipes, YUI, BOSS, and several other existing Yahoo technologies into Yah…
Mozilla and Yahoo would make a good strategy, considering Google/Mozilla relationship has cooled in recent years.
I like your Github/Meteor suggestion as well, albeit unlikely that either of those companies would take money or influence from Yahoo.
Also don't think Marissa Mayer has the vision for these sort of changes, although hope I'm wrong on this.