This question came up recently in a discussion that was proposing a "Workers Party" for Namibia after the last election.
I had to think about it for a while, especailly as I am a SWAPO member.
But then inspiration struck, if we want to tackle a political problem, why not look at how this has been done by consumer activitists (such as Ralp Nader in the USA). The most common denominator for people is their consumer experience in a society. Nader considered launching a third party around issues of citizen empowerment and consumer rights. He suggested a serious third party could address needs such as campaign-finance reform, worker and whistle-blower rights, government-sanctioned watchdog groups to oversee banks and insurance agencies, ...
I suggest that should a "third-party" emerge it would only be viable if it had a coherent political platform.
What should this platform be based on?
Four things, namely the social movements, the peace movement, the civil rights movement, the environmental movement, and the labour movement. (Often referred to as the "green movement".)
This would be a political platform I could follow!