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Yes, I like your idea about passing an object as the parameter of theme functions. However, in XOOPS2, there will be no theme functions at all, but all the values for presentation are assigned to template variables via the Smarty global object like $smarty->assign('name', $value);.
Yes, the old themes will still work fine in XOOPS2, but since the Smarty template system will be the primary theme system we will be using, it is unlikely that we will be doing any modifications on the current theme system.