A work in progress abstraction that will allow user meta for all users including guests. Please contribute.

datastore.php

<?php
namespace storyteller;

/**
 * This is a class designed to be used in WordPress Plugins and Themes where for
 * whatever reason, the designer wishes to track a huge ammount of per-user data
 * and keep it well ordered and accessable.
 * 
 * I am posting this in an incomplete state; in the hope that further developers
 * might have ideas that will finish the project for the good of the community.
 * 
 * This is part of a wider project to use WordPress for interactive fiction as a
 * form of storytelling. Thus the namespace.
 * 
 * Datastore is an abstraction layer to store data within user_meta or elsewhere
 * against guest_cookie (in site_meta) which requires garbage collection.
 * 
 * datastore is intended to query the data quickly.  For example via a number of 
 * shortcodes and assorted API yet to be created. It should be possible to ask a
 * question about the data without any awareness beyond getting TRUE or FALSE as
 * a return value. Thus, datastore::test_var(...);
 * 
 * @package     storyteller
 * @subpackage  storyteller.datastore
 * @author      Matthew Brown <https://twitter.com/lordmatt>
 * @copyright   (C) 2019 Matthew Brown.
 * @license     http://www.gnu.org/licenses/old-licenses/gpl-2.0-standalone.html 
 * 
 * VERSION: 0.1
 * 
 * =======
 * History
 * =======
 * 
 * 0.1 - the basic IO into and out oft he array has been covered but more work's
 *       needed for the store and load methods.  
 * 
 * =================
 * Unsolved problems
 * =================
 * 
 * Where to put guest user data?
 * -----------------------------
 * 
 * All signs point to abusing existing API as a very bad idea.
 * <https://wordpress.stackexchange.com/q/344323/109240>
 * 
 * This may require the creation of a session meta_table of some kind.
 * 
 * 
 * Useful reading
 * --------------
 * <https://codex.wordpress.org/Metadata_API>
 * 
 * =======
 * License
 * =======
 * 
 * This file is part of storyteller/datastore.
 * 
 * datastore is free software: you can redistribute it and/or modify it under 
 * the terms of the GNU General Public License as published by the Free Software 
 * Foundation, either version 2 of the License, or (at your option) any later 
 * version.
 * 
 * datastore is distributed in the hope that it will be useful, but WITHOUT ANY 
 * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR 
 * A PARTICULAR PURPOSE. See the GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License along with 
 * datastore. If not, see http://www.gnu.org/licenses/.
 */
class datastore {
	
	private $data=array();
	
	protected function update_flag_group_count($group){
		$count = count($this->data['flags'][$group]['flags']);
		$this->data['flags'][$group]['count'] = $count;
	}
	
	public function set_flag($flag,$group='global'){
		$this->data['flags'][$group]['flags'][$flag]=TRUE;
		$this->update_flag_group_count($group);
	}
	
	public function get_flag_group_count($group='global'){
		$this->update_flag_group_count($group);
		return $this->data['flags'][$group]['count'];
	}
	
	public function add_to_var($var,$change=1,$group='global'){
		if(!isset($this->data['vars'][$group][$var])){
			$this->data['vars'][$group][$var]=0;
		}
		$old = $this->data['vars'][$group][$var];
		$this->data['vars'][$group][$var]=$old+$change;
	}
	
	public function get_var($var,$group='global'){
		if(!isset($this->data['vars'][$group][$var])){
			$this->data['vars'][$group][$var]=0;
		}
		return $this->data['vars'][$group][$var];
	}
	
	public function flag_is_set($flag,$group='global'){
		return isset($this->data['flags'][$group]['flags'][$flag]);
	}
	
	public function test_var($var,$val,$comp='=',$group='global'){
		// $comp <= < = > >= !=
		switch (trim($comp)) {
			case "!=": return $var != $val;
			case ">=": return $var >= $val;
			case "<=": return $var <= $val;
			case ">":  return $var >  $val;
			case "<":  return $var <  $val;
			case '==':
			case '===':	
			case "=":  return $var == $val;
		default:       return false;
		}  
	}
	
	public function load_data(){
		# if user logged in
		# load user_meta('story')
		# if guest , get cookie key , find store from ????
		# I think that I have cracked it: <https://gist.github.com/lordmatt/3bd8f7787fbbe0950f9228dec92f2f8a>
	}
	
	public function save_data(){
		# if user logged in
		# set user_meta('story')
		# if guest , get cookie key , find store in ???? update ????
		# I think that I have cracked it: <https://gist.github.com/lordmatt/3bd8f7787fbbe0950f9228dec92f2f8a>
	}
	
}

I think I have an answer in the form of a one-file plugin that I wrote called Guest Meta.