Hippo defense
The Hippo did see one game at a World Championship match, so it is playable.
I think it's general setup isn't very appealing to most players, and never really got traction.
With high level chess engines always expanding chess theory, it's only a matter of time before people start having them set up to play rare and oddball lines, out of curiosity.
I think it might be decades before even all the regular lines are vetted though. Mainly because chess engines will be improving for years to come, and stronger engines will keep adding new theory to the strongest lines.
Only when all that is vetted out, will programmers start having opening books focused on solid, but less popular lines, then after those are vetted will programmers start looking at playing rare and oddball lines.
One thing I've noticed is that whether an opening is played at the WC level has absolutely zero to do with whether it can be played at our level. I am for example playing th e Danish Gambit and Halloween gambit with great success because my opponents always make mistakes. It would be unthinkable to play these lines at the WC level.
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What do you think about it?
https://www.newinchess.com/media/wysiwyg/product_pdf/9081.pdf