The reason why Keiko decided to take up a job in division 4 was not as simple as everyone thought other than working on his new Kido skills, Keiko also used this as an opportunity to collect souls of shinigami for himself. By making a name for himself, he could get involved in more cases many of whom were critical and didn't have many chances of recovery and death as a result, using this most he started feeding his Hogyoku's which he had created, in the beginning, he only feed one Hogyoku out of 13 which he had and started powering it. After a few days of experimentations, he understood there was no upper limit to how much an Hogyoku can be feed and charged, unlike reality stone from marvel which consisted of unlimited power. Using this as a base he started feeding other Hogyoku's too, with a new mindset to create an even better version of Hogyoku which could provide unlimited reality wrapping ability like reality stone.
With his dismissal from division 4th, Keiko went back to Royal palace and reported in the situation while also stating all his observations in regards to defense, attack power and other things related to soul society. Soul society, in general, served as the first layer of defense for the soul king thus were an important part of the system thus Ichibe and others indirectly paid attention to its activities without directly being involved in it. Ichibe and others were familiar with the situation but after hearing Keiko's they knew that soul society is weakening by the day and decided to follow Keiko's plan to let the next major thing happen out to test the overall strength while also letting Keiko sit back and observe the situation thus permitting him to stay for more longer time within the soul society.
Keiko, after completing all his duties and gaining the permission, resumed all his operations again which he midway when he was called into the soul society, he started with collecting Hollow for his collections and started using various methods to envolving from simple Hollows into Menos, which was formed by the collection of hundreds of normal Hollows, he was truly interested in the their evolutions, first, to be created were Menos Grande also known as Gillian, the likes of which was Ichigo and Rukai fought will fight in the human world, Keiko's interest was within the ability of Negacion which they were able to produce by which they could rescue their comrades. Keiko made them use this ability while trying to penetrate the light square in various ways, he first used his eye ability of Kamui by which he easily was able to enter and exit the square while harming the Hollow inside which under the Menos protection, then he started using the space fluidity but failed to either repeal the beam or crack the space in it to enter, thirdly he tried to use speed to already inside the space where the person to be protected was already in but felt that the energy was pushing him out while making a protective space around the person meant to protect. Lastly, he tried to use Kido art he had created but still failed which fascinated him even more. Then he started with the creation of Adjuchas who had to be feed regularly or would lose their mind, Keiko using his seal method found a way to make this losing mind process nullified while also adding more spiritual energy into them, each Adjuchas had unique individual powers and Abilities which ranged from extream speed, strength mind control to other things. Keiko wasn't much interested in this as these types of Hollows only were interested in hunting not developing their ability.
He started the final phase to create the highest grade of Hollow their was Vasto Lorde which no one knew how were created. Keiko first started by feeding the Adjuchas normal hollows but didn't get any result thus decided to much the matter more and started feeding Adjuchas to Adjuchas after sacrificing thousands of Adjuchas 2 out of 10 Adjuchas which Keiko was feeding developed and changed into Vasto Lorde, part hollow appearance while having Part human appearance too. Keiko decided to look more closely into what makes those individual specials, he first started with the type and nature of the soul in question and also the type and level of energy which both had. One Vasto Lorde being more powerful than others while both having more consciousness to make sensible reasonable decisions. The unique evolution process was involved which lead to their coming into being, Keiko with his deep research started modifying the souls of the other 8 subjects which he had been feeding, little by little he had 10 Vasto Lorde for himself.
Happy with his work and knowledge, Keiko decided to go even further and change/evolve Vasto Lorde further. He started by creating more Vasto Lorde in the beginning and created more than 50 for himself. Then he chose the Vasto Lorde which showed the most promise depending on both their power level and soul strength he introduced the power of Hogyoku to the selected few. Slowly changing them and making an envolved Arrancar from Vasto Lorde for himself.
He continued his practice and created around 20 envolved Arrancar with Captain commander strength while 50 Vasto Lorde with above captain level.
With his little army in place, he started another project, humans. One of his Arrancar who specialized in spying informed Keiko that Rukai Kuchiki has left the soul society and has been sported in the area near Ichigo Kurosaki's home.
Keiko amused at the information decided to take some time off, ordered his Arrancar to go and kidnap some humans of different, ages, nature while also bringing animals of one kind each.
Keiko prepared to visit the human world for himself, he wanted to watch the entire process of which the Hogyoku which Urahara embedded in Rukia gives power to Sado and Orihime. He also had the option of just going and taking the said Hogyoku for himself but he was more interested in the progress of the story.
Keiko started stalking Rukia as she did her duty to investigate unusual Hollow activities in the area and finally show her meeting the weird Shinigami seeing kind Ichigo Kurosaki.