Basic Idea. What would the world be like today if Napoleon's empire didn't fall? What would he need to do(or not do) in order to accomplish this?
So if Napoleon and Metternich actually finds together at their meeting in mid 1813, Prussia and Russia would have to fight the autumn campaign alone, with a great risk of a decisive defeat. If/when that happens, Napoleon can turn his focus on Spain - and bye-bye Wellington.
I'm not totally sure Napoleon would have beaten off Wellington in Spain so easy actually.
Napoleon was a great strategiest and tactican but his real strategic sense mostly worked where he could fight the enemy in an open battle. The Pennisuelar war was from the spanish side IIRC a guerilla war where Napoleon couldn't fight them in a real battle.
Correct me if I'm wrong.
Russia, the UK, and Austria refused at accept permanent Napoleonic hegemony over Europe. Napoleon refused to accept anything other than permanent French hegemony. His invasions of Spain and Russia demonstrate the amazing (overreaching) breath of his ambitions. He seems to have always believed himself one more victory away from the final blow. The problem was that arrayed against the UK and Russia, there was no military solution.
To be honest, the Penninsula was one of Napoleon's greater blunders, but it could have been easily won. The french tended to have numbers on their side, and the Iberian guerillas were rarely a threat as long as the Allied Field armies could be eliminated. And as late as 1812 the country was for the most part in French hands. If Napoleon had taken the time (or been given it) to focus on events beyond the Pyrenees, or if his Marshalls had not squandered the war effort, a victory in the penninsula even as late as Vitoria is easy to concieve.
You're right that the Peninsula War could have been won for France, but I think you underestimate the contribution of the guerrillas and the difficulty Napoleon would face. For most of the occupation of Spain, France had 300,000 troops in Spain, yet was only able to utilise about a third of them at any one time. The guerrillas weren't great when they faced defended locations, and could be dispelled with some ease, but that was inconsequential really. The Spanish guerrillas may have acted like irregular cavalry, but their ability to disappear and reappear was rather indicative of the Vietnam War, as was the way that the locals often protected them from being found and the way that the occupier often had to persecute or slaughter villages of innocents to try to shake the guerrillas into giving up. The guerrillas were just very, very effective at appearing from nowhere and cutting the French supply lines, forcing the French to post garrisons in the most inconsequential places and send battalions running all over the place. For this reason the French can't concentrate their forces as you would expect them to if you just read the numbers on paper available. And yes, the French still had a numerical advantage on the British/Spanish/Portuguese armies, but I think you'd have to expect that Wellington would take overall control of any and every battle against Napoleon, and with Wellington being a man very much inclined to picking his battlefields and letting his opponent do the hard work, the end result of some crucial battles may not be unquestionably French victories. The British also have the Lines of Torres Vedras to fall back on, where Wellington was happy to sit and wait for a better moment, and with Napoleon a very impatient character I can see him giving up on Spain for better places to conquer and win glory, leading to repeats of the OTL French reverses in Spain.
I'm not saying that the French would be hard-pressed to actually win the Peninsula War, but I'm not convinced that Napoleon would have the patience to comprehensively clean up and win the campaign, and I think if he gets bored and wanders off, the end result is entirely uncertain.
The British won the Napoleonic wars because they had all sorts of advantages - advantages that don't go away even with no Peninsular war.
1. They Royal Navy. The RN after Trafalgar was unmatched, and, I'd argue, unmatchable. Yeah, the French could build more ships. But could they man them with experienced sailors and admirals? The Royal Navy keeps Britain fed, supplied, the Empire in line, and keeps the French from expanding their empire out of Europe. No French Algeria, or Morrocco, or India.
2. French population. However you look at it, the French were a minority within Europe. Eventually, others were going to resent French dominance, no matter how 'nice' the French were. The revolutions of 1848 in 1825, anyone? Nationalistic asperations were inevitable.
3. Other allies. If the French are looking tough enough, the British can always find new allies...say the USA. The early USA would have been little more pleased with a hegemonic French Empire in Europe than the British. With a bit of common sense both the USA and UK could have their 'special relationship' about 100 years early. Even foregoing military cooperation, the USA would be only too happy to sell stuff to the British, especially if the Europeans were unable to because Nappy ruined their economies.