In the case of Misty & Brock during Johto the lack of rivals is not a problem, rather the symptom of a bigger issue with their storylines or the lack thereof. Even if recurring characters were created to oppose them there was nothing to be in opposition about on a consistent basis. During Kanto & Orange Islands it really didn't hurt to have characters of the day or Team Rocket take on that kind of role in the context of any one episode.
They had storylines, they just werent fleshed out enough.
Misty story was to become water pokemon master. Presumably prestigue title which was more thoroughly touched upon during Whirl Islands. She admired E4 Lorelei idolizing her and wanting to become best water trainer in world. Had complex of inferiority imposed by sisters. Fears and flaws such as fear from Gyarados, judging pokemon by look rather than inner strength etc.
So i imagine writers could play on that card by giving her rival which is driven with same passion and care for water pokemon like she is. Want to catch as many as he/she can learning their secrets and become top water trainer, possibly E4 member apprentice(since they are best mono trainers in world).
With Misty herself trying to remove competition out of her way and gain E4 sympathy leaving better impression on him/her than rival could for place of aspprentice.
Providing opposition and obstacle in confronting Misty views and challenging her skills or attainment. Competing against each other in Whirl Cup and chain of other water related tournaments expanding on water master title and what it entails being learning experuence for both her and rival, test out your bond with pokemon and prove yourself to other experts.
Marina from Orange Islands was one of such candidates imo being devoted and passionate about this specie just as much. But renowned trainer like Trinity or Harrison from Whirl Islands introduction much earlier in story would fill the bill nicely too.
Staggering her confidence and security as water trainer feeling despondent after harsh defeats and beginning to question if her life calling of being water specialist who wants to learn how to use them to full potential was really right choice? And if she should just quit on it altogether?
Going through selfsearching and realizing how she loves water pokemon too much to give up, with new strength and motivation found in meantime giving her edge in proving opposition wrong about her talent and abilities.
There is also that hotheaded trainer Egan from Johto which belittled Misty skills and argued over fire types being more useful than water ones inciting arguing and spirited adversary between them.
So writers could had introduced trainer from different type challenging Misty beliefs about water and qualities of them providing nice subplot through which she would have to learn how to develop adequate counter moves vs pokemon of other types which have advantage such as electric or grass. Or effective strategy to overcome inferior position in which her water pokemon could be put in.
For Brock since he wanted to become top pokemon breeder in world, writers could introduce character like Suzie who is driven with same support and morals about how should pokemon be raised. And be kept in top shape.
Challenging Brock knowledge and methods of preparing pokemon food, preparing medicines, way he groom or excercise with them. Imposing his own ways of raising pokemon, healing them and extracting their full power on surface without battling as more "superior ones" and rewarding as far as pokemon breeding goes.
Giving to Brock hurdle, constant which would bring insecurity and anxiety from his character about his own breeder methods. Trying through battle or sport like activities to determine whose pokemon is better raised and taken care of through examinating their speed, glow of fur, physical strength, agility etc.
It definitely had potential imo.
