If G5 is to be a continuation of G4:
Twilight's reign over Equestria was stable and peaceful for decades. Although eventually, Twilight would outlive all of her friends. The land was peaceful although without a few problems here and there. Twilight would rule for decades under peace and friendship. However that wouldn't last forever, eventually a group of Unreformed changelings had heard of the legend of Queen Chrysalis, although they had elected their own Queen and survived, none had been as notable as Chrysalis was. Eventually they would break Chrysalis and Tirek, Cozy Glow out of their stone prisons. Queen Chrysalis would return as the Queen of the Changelings and the 3 villains would team up.
Eventually they had planned to take over the world again but learning from their mistakes, Lord Tirek only drained a few ponies and the 3 villains fled beyond Equestria. Twilight, without her friends was very angered and worried over this. She was worried that everything she had lived to achieve was falling apart, and that she had failed Celestia and her friends. This worry and fear corrupted her, transforming her into Daylight Sparklenight (I don't like that name but it will have to do, a Nightmare version of Twilight). She had vowed to hunt down those villains and defeat them.
As Twilight's worry grew, she grew more and more mad with power and control. Eventually she transformed into a dictator who was heartless but also very sad, having lost her friends to time. A few years after that, she had found herself in war with the new Changeling Army under Chrysalis who had defeated the Changeling Kingdom and Conquered the unreformed Changelings, eventually transforming them back into what they were. The Descendants of Thorax's leadership were killed, and the Changeling Empire under Chrysalis's rule returned.
While Tirek lead his own army of monsters of Tartarus to attack the ponies. While him and Chrysalis had an alliance, they formed some sort of combined monster-Changeling army and stormed Equestria. A massive battle ensued, eventually Twilight or whatever her Nightmare evil name would be would battle Chrysalis and Tirek, while Cozy Glow had set up her own plan that would eventually backfire on her. The battle was fierce, but there was no conclusive winner.
The armies would grow tired and the war was a stalemate, Twilight was forced into an agreement with Chrysalis and Tirek. Chrysalis and Tirek would withdraw their armies from Equestria, but Twilight would not pursue them further. This new 'peace' would last for decades more with occasional clashes.
Eventually, a group of Enlightened Wizards had enough of Twilight's rule and they rebelled, they stood true to Celestia's old ways (almost like Cultists), and eventually overthrew Twilight...using the elements of harmony (they found new elements). Twilight was banished into the stars as a constellation and the wizards themselves collectively ruled Equestria for a time, freeing it from tyranny.
This freedom granted them the power to rise the Sun and Moon and Equestria became what it once was more or less. Eventually the most powerful one would lead Equestria as it's first King. The King ruled with peace and fairness, while defending the land from threats otherwise and became an Alicorn, eventually finding his love who became a Queen herself.
Tirek and Chrysalis would grow old and more powerful, becoming something of Legends themselves but still ruling their Empires before passing on, leaving their descendants to rule.
Eventually, the Queen would find herself a student, and that is where the story would continue.
If G5 is all new:
If it is all new I would like to see Alicorns of various elements, seasons, more than just Sun and Moon. Almost like gods with their own followers, temples and shrines.