I don't know if you'd want a product to replace that template, because some people might have custom styles (and there's also the issue of multiple styles and mobile styles). I'm not sure of the right way to handle that. But one thing you could do is name your new template something else, then arrange for your template to be used. The code that uses that template is in functions_forumlist and looks like this:
PHP Code:
// build the template for the current forum
($hook = vBulletinHook::fetch_hook('forumbit_display')) ? eval($hook) : false;
$templater = vB_Template::create("forumhome_forumbit_level$depth$tempext");
$templater->register('childforumbits', $childforumbits);
$templater->register('collapseimg_forumid', $collapseimg_forumid);
so you could have a plugin using hook forumbit_display that changed $depth or $tempext so that it was using your new name.
I guess this still causes issues if someone has a custom style, so I'm not sure if this is really any better.