Do I have to write my own {TABLE} plugin for colspan and rowspan to work?
Interesting and really good question!
Starting a new table plugin/syntax would certainly be an option (it's the "wiki way"), but i think it would be better to add new functionality to the existing wiki table syntax, and/or the {FANCYTABLE} plugins.
Doesn't this approach in fancytable almost do what you need already?
Another thought to bear in mind is that we have a long term goal to move to markdown eventually, as planned here WYSIWYG and Markdown, so maybe it's worth seeing how md does this? (if it does)
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