Hello, I've been having a pretty nice challenge playing some "hardmodes" for the grand campaign that I've made up/found from some other people on making grand campaign interesting without having to use mods, note that the difficulty I've used is VH/VH:

Great Britain Hardmode: This one I've played the most and really enjoyed, doing it is fairly simple. Basically, wait it out untill France conquers the map (or most of it) and then have your fun liberating Europe. Some things to note is that you will have to wait and not expand, in fact I highly recomend giving up Gibralter early instead of fighting for it (either leave it on turn 1, or hold on to it for a year or two to expand your fleet or trade ships, it helps to leave a spy in Madrid to set up a spy network to see if France starts sending armies there). There reason why I say leave Gibralter is due to the fact that France will start sending troops to "assist" Spain in case you start to invade Madrid. If you wait long enough, they WILL start sending many stacks (I held Gibralter once untill 1811, and they have 4 full French stacks). This takes away from their ability to take the rest of Europe, so it will take MUCH longer than needed if you hold Gibralter. In addition, you shouldnt take any territory in Europe (including Rennes), this is because it will cause France to send their Eastern troops to counter it. Before France becomes all of Europe, you can survive with one stack in London, and then focus on your economy, and navy, etc. Its your choice whether to blockade and stop France from being a naval threat. I personally didn't, which made it VERY challenging, and stressful on your economy at times. In addition, in order to make France even tougher, I payed them excess cash and gave them tech's. I also payed any liberated country of mine to join the war, so it wasn't unfair (I can attack, they can't).

France Hardmode: this is extremelly fun, basically, all you do is turtle the entire campaign, untill you feel like fighting. Dont help allies. Dont expand. You'll be fighting many fronts, and have to struggle to play a constant balancing act (Expand economy, expand armies and send to needed fronts, expand navy and protect your trade). I've played this till 1810 and still haven't expanded. It gets very difficult. I didn't blockade any British ports, I wanted them to have fun making landings. (You might have noticed so far I like rooting for the opposing team for some reason).

Russia Hardmode: Basically, similar to British, sit and wait till France is completely at your doorstep, then fight back, or see how long you can defend! Don't expand! This is particularly challenging since Russia's econ isn't great, nor is their military. Prepare to defend your very long border, while France fields waves of armies alongside the Ottomans, remember to take advantage of the winter.

There are some others, such as turtling as Austria or such. Although I really think these three are the best. Although, thinks such as multiplayer grand campaign, Prussia and Austria vs Russia France Britain Spain is VERY challenging, particularly if you just want to turtle.