this is a bad idea. It should be a prototyped array node if you want to allow any option, not a variable node. You don't want to allow anything else than arrays for options.

and using a variable will be very unfriendly to XML users, as it is not possible to define an associative array in XML (Symfony has a way to deal with it for prototyped array nodes).

the rule of thumbs is: if you start using a variable node, think twice about it. Most of the time, it is a bad choice.