Tx_Pnutility_View_VariantView

SomeController.php

<?php

class Tx_Polarmedia_Controller_MediaController extends Tx_Extbase_MVC_Controller_ActionController {
	/**
	 * @var defaultViewObjectName use custom extended Tx_Fluid_View_TemplateView
	 */
	protected $defaultViewObjectName = 'Tx_Pnutility_View_VariantView';

	public function teaserAction() {
		$variant = $this->settings['variant']); // or anything else(userState...)
                // $variant = 'mini';
                // -> Teaser.Mini.html
                // You must not(!) have Teaser.html in the Temaplte Directory or you will run into an exception(!)
		$this->view->setLayoutVariant($variant);
	}

?>

VariantView.php

<?php
/* 
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 * This is a little adoption to  Tx_Fluid_View_TemplateView
 *
 * its purpose is to allow differnet variants of a layout (very similar to @format)
 *
 * use cases could be: differnet templates for different colPos or based on any other criteria
 * Just a different layout/display of the very same data
 *
 *
 * @author pn
 */
class Tx_Pnutility_View_VariantView extends Tx_Fluid_View_TemplateView {
    //put your code here

	/**
	 * we add another param to resolve the name of the template
	 * 
	 * same action/sameformat but different temnplates by using the variant param
	 * 
	 * usage: Controler->view->setLayoutVariant('foo');
	 * 
	 * @var string
	 */
	protected $layoutVariant = NULL;

	protected $templatePathAndFilenamePattern = '@templateRoot/@controller/@action.@variant.@format';

	public function setLayoutVariant($layoutVariant) {
		$this->layoutVariant = $layoutVariant;
	}

	/**
	 *
	 * overwrite/adopted for @variant
	 *
	 * attention: due to the currentw ay of ActionController->resolveView() wee need to utilize initializeView() [seee below] to set templateRootPatg ourselfs
	 *
	 * Resolve the template path and filename for the given action. If $actionName
	 * is NULL, looks into the current request.
	 *
	 * @param string $actionName Name of the action. If NULL, will be taken from request.
	 * @return string Full path to template
	 * @throws Tx_Fluid_View_Exception_InvalidTemplateResourceException
	 * @author Sebastian Kurfürst <sebastian@typo3.org>
	 */
	protected function resolveTemplatePathAndFilename($actionName = NULL) {
		if ($this->templatePathAndFilename !== NULL) {
			return $this->templatePathAndFilename;
		}

		$actionName = ($actionName !== NULL ? $actionName : $this->controllerContext->getRequest()->getControllerActionName());
		$actionName = ucfirst($actionName);

		$paths = $this->expandGenericPathPattern($this->templatePathAndFilenamePattern, FALSE, FALSE);
		foreach ($paths as &$path) {
			// *************
			// adoption start
			if ($this->layoutVariant !== NULL) {
				$path = t3lib_div::fixWindowsFilePath(str_replace('@variant', $this->layoutVariant, $path));
			} else {
				$path = t3lib_div::fixWindowsFilePath(str_replace('.@variant', '', $path));
			}
			
			// removed upstream fallback to lowercase
			$path = str_replace('@action', $actionName, $path);
			if (file_exists($path)) {
				return $path;
			}
			// adoption end
			// *************
		}
		throw new Tx_Fluid_View_Exception_InvalidTemplateResourceException('Template could not be loaded. I tried "' . implode('", "', $paths) . '"', 1225709595);
	}

	/**
	 * wee need to apply the TypoScript-Setp again ourselfs
	 */
	public function initializeView() {
		parent::initializeView();
		$extbaseFrameworkConfiguration = Tx_Extbase_Dispatcher::getExtbaseFrameworkConfiguration();
		if (isset($extbaseFrameworkConfiguration['view']['templateRootPath']) && strlen($extbaseFrameworkConfiguration['view']['templateRootPath']) > 0) {
			$this->setTemplateRootPath(t3lib_div::getFileAbsFileName($extbaseFrameworkConfiguration['view']['templateRootPath']));
		}
	}
}
?>

I started a discussion about a solution ready for TYPO3 8.7 on https://stackoverflow.com/q/44568353/7173655