Safest option would be to turn Valve into a foundation, similar to Zeiss: The company owns itself thus can't be bought out, statutory stipulations go along the lines of "Make sure we don't go bankrupt, do great optics, funnel some money to the University of Jena as well as less well off workers in the city". No grand charitable aspirations besides being a good citizen as per 1846 bourgeois values (though that'd be Gabe's choice), otherwise just continue as they've been doing. Make sure all stakeholders (incl. gamers, or at least Valve customers) have standing to enforce those statutes.