For some peopel a trike would for sure be the best. I know that for quite a lot of peopel the price tag would be a problem and also trying to build one for themself or a friend could be a challenge.
I just want to tell about the guy in my area that made a living out of adapting ordenary bikes to peopel w special needs. He often puts heavy duty "training wheels", I think 12" and 16" and they are atached with really strong suspended "arms".
For peopel who can use this system this could be a fairly cheap but good solution since the bike could be one that is already owned and a friend or a relative could do the fitting without much knowledge.