Before the War of the Five Kings, the last major war definitively caused by a succession crisis had been the War of the Ninepenny Kings, where Maelys the Monstrous, previously thought to be the last Blackfyre, claimed the Iron Throne. That war, however, was never even fought on Westerosi soil. The War of there Three Stags, however, had been insanely traumatic for the Seven kingdoms; whole regions were devastated, and humanity had literally almost gone extinct due to the disunion in the face of the others and Euron. There was a general consensus across Westeros that the succession simply needed to be officially handled. Thus, in 310, Myrcella released an official set of succession rules to the throne. They essentially stated that the eldest son of the current monarch should inherit the throne; if there were no sons, then the eldest daughter would inherit. Crucially, the loose precedent of the Great Council of 101 was officially done away with; there was some ambiguity because that precedent had stated that no woman could ever sit on the Iron Throne; however, this was challenged when the Blacks won the Dance of Dragons. Some ambiguity remained, however, because Queen Rhaenyra had never been the unchallenged Queen. Now, however, Myrcella solidified that daughters came before brothers. Further, grandchildren also came before brothers. While this would not prevent wars of succession entirely, it would make them less prevalent because the question of succession itself was less ambiguous.
Myrcella then began on what would become a general theme of her reign: empowering the monarchy. The main step to this was centralizing the Crownlands; the lands taken from the Reach and Stormlands had already been stripped of their previous owners and transferred directly to the crown, and Myrcella would use this as a power base to further disempower the houses of the Crownlands. For some, such as among others Houses Massey, Celtigar, and Velaryon, she had the excuse that they supported Stannis and in the case of the latter two Aegon. Houses Stokeworth and Thorne, meanwhile, were sufficiently weak as to be strongarmed into agreeing to Myrcella's centralization. Only the house of Rosby was foolish enough to resist, and they were quickly put down(upon reading a note from lord Rosby declaring that on their victory the Lords of the Crownlands would decide Myrcella's fate, the Queen remarked "I am the Crownlands!") by King-consort Jon, by now recognized as one of the more able generals in Westeros.
By 315 Myrcella's authority in the Crownlands was unchallenged. She used this power to create the first professional army in the history of Westeros. Open to any man between the ages of 18 and 35, the Royal Army was essentially an evolution of the Goldcloaks. However, while the Goldcloaks were notorious for their corruption and lack of training, this new Royal Army was extremely well trained and answerable only to the Monarchy; the Queen could appoint and dismiss a General at will, and any and all recruits were subject to strict training. The job also paid well, better than most of the jobs a commoner could enter that didn't involve service to a noble family. Another factor differentiating the Royal Army from the Gold Cloaks was the fact that the Royal Army was recruited from across the Crownlands, while the Goldcloaks were limited to King's Landing. This had the impact in the long term of making the Crownlands the single most powerful militarily of the regions of Westeros; no longer would the Monarchy need to bow and scrape to any Lord Paramountcies whenever some pretender threatened the throne. Their professional army were also some of the best infantry in the world; they weren't as good as the Unsullied, but they could reliably defeat any equal number of feudal levies in a straight fight, and could even occasionally perform very competently when outnumbered. At their inception, however, they were only 5,000 strong, though they would swell in numbers later to 20,000. This expansion, of course, would need money, and no attain this Myrcella ordered the launching of great trading fleets.
The trading fleets would sell goods unique to Westeros(for example, Arbor Red and Dornish gold, and frozen meats of species native to Westeros such as Aurochs) to nations far away from Westeros such as Qarth, Yi Ti, and even Asshai. These fleets were commanded by second sons of Westerosi lords, eager to prove themselves. Asha Greyjoy and Trystane Martell were two notable commanders of the fleets. The impact of these fleets was twofold: firstly, it vastly improved Westeros's financial situation, and allowed for the creation of the Royal Army, and secondly, it improved Westerosi contact with the outside world. It was through these connections, for example, that gunpowder came to Westeros from Yi Ti, though it wouldn't be used militarily until after Myrcella had died.
By 330, the Westerosi Royal Army had become a formidable fighting force, giving the monarchy far more clout in the affairs of the Kingdom. It was at this point that Myrcella turned her attention to one last thorn in the side of Westeros: the Stepstones. This archipelago had long been a haven of pirates and rebels, and they were occasionally a threat to the monarchy. For example, the pirate lord Salladhor Saan had supported first Stannis Baratheon and then Aegon in their wars. These islands simply could not be allowed to persist as an independent entity. Thus, in 330 Myrcella would declare her second war, the Stepstones War.
The war was mostly fought by Dornish forces, as Arrianne Martell planned to integrate the archipelago into her principality. However, the Royal Fleet managed to defeat the majority of pirate navies, and the Royal Army did cut its teeth in this war. Especially notable in this war was the heroism of Ser Brandin Stark, the White Wolf, and Lord Commander of the Queensguard. He was the first off the ships when storming Torturer's Deep, and cut down dozens of pirates with the Greatsword Ice. Ultimately, after around a year, the islands had been conquered, and annexed into Dorne. Arriane made Obara Sand lady of the Stepstones, creating the new House Viper. Obara would be rather successful in purging the Stepstones of piracy, and they were fully integrated into Dorne by the time of her death.
The rest of Myrcella's reign was peaceful. Her son Steffon Baratheon was married to Princess Rhaella, daughter of Empress Daenerys, creating the House of Baratheon-Targaryen, whose banner of a Stag and a Dragon flies over the Red Keep to this day. She refilled the treasury, making the Crownlands prosperous. Finally, she died in 350 AC, after a reign of 52 years. She saved Westeros from the Others, Euron Greyjoy, and, perhaps most significantly, itself, by centralizing the realm and stabilizing the line of succession. Of course, there would be other civil wars(including the infamous 15 Years War) but they would be much less frequent. Myrcella found Westeros a backwater and left it poised to become the hegemon of the West, the Free Cities essentially unwitting as to how outclassed they truly were. She is worthy of her place as one of the greatest monarchs on the Iron Throne.