A rival for what? She didn't even have a goal that required a rival.
That's not necessarily true. In big place like pokemon world you would think there would exist other people who specialize in water pokemon just like Misty wishing to achieve mastery with them. And we saw quite a few of water trainers such as water trainer Marina, Andreas, Trinity and Harrison Misty fought against. Sharing that enthusiasm and devotion for water species.
Needless to say it was showed through Whirl Cup and whole title "Alpha Omega of Water Pokémon!"how there exists tournaments and tests Misty has to enter and win in order to come closer toward her dream. Being like stepping stone in accomplishing title of becoming water master.
Concept which wasn't explored enough, but there were set foundations through this tournament, Seaking contest, water race etc.
So when counting all those gym leaders and E4 members specializing in one pokemon type like Misty, even champions like Wallace. All those water trainers Misty met, it would be rather logical to assume how there exists other people who just like Misty wish to become type masters(we already learned about one;Iris)or strive toward reaching E4 level of strength and recognition(something Misty again wish to reach as well favoring Lorelei).
I have to say giving Misty rivals like Andreas would have done lot of good for her journey . For once her dream would be explored more, there would be provided some opposition and tension through arrogant and disrespectful trainers like Andreas was. Challenging Misty beliefs and temperament nature. As well make her reconsider why she trains only water pokemon and where she does really stand in world full of other water trainers driving her forward. Through battling each other, building on interactions, entering tournaments similar to Whirl Cup etc.
Andreas was definitely perfect material for rival, although Marina from Orange Islands who had lot of enthusiasm and interest for type Misty adores as well.
