I don't thing it is possible to unite all martial arts. (Though I think that it is a reality that they are all just translations/interpretations of the same thing - could try to explain but don't have time at the moment).
What can be done, is to unite different Arts towards a single goal and in doing so improve the awareness of each art/group as to the similarities all arts share. Also to the potential of sharing knowledge and expertise - 'only a fool does not want to learn more'.
Six years ago I organised a Martial arts festival to raise funds for Cancer Research UK. The first year I contacted 10 different arts who freely gave up their time and expertise to take part in a day long demonstration of their arts in front of an audience of about 300 - we raised around £1000.
Last year ( the 5th) 14 different arts/ groups took part raising £2600 This year will be the sixth annual festival, planned for May, and I have Marts groups already contacting me wanting to take part -
A single goal!!
But as an added bonus there has been created a 'network' of arts who as martial artists and also friends - are only too willing to get involved in basically any way they can to support each other. Whether it be raising funds for charities, doing demo's/seminars for each other giving advice, sharing info, contacts etc, etc.
I have myself delivered Aikido seminars for Martial Arts 'friends' who study Kung fu and Karate - I learned a lot just by being there with 'other arts'.
(In the near future I have arranged for a senior Kenjutsu sensei to deliver a seminar for my Aikido class - I know myself and my students will gain a tremendous amount from this seminar).
And I am hoping to invite a very senior Karate instructor along to share his art with us in the near future.
Mutual respect, friendship, co-operation and an open, mature outlook on fellow M Arts.
Nearly forgot - the interest in the 'Martial arts' in and around the city where I live also increased.