there was a thread about this a while while back... but I'll give my opinions on the matter, and what I'd like to see.
they should have multipule rivials... much like Wally and Brendan/May.
however, there are 6 different trainer sprites... when you begin the game, you choose your gender and then you name your character. Then you choose the rivials gender. Which has 2 different sprites from your character, a male and female one. Then you choose your rivial's name. As you progress through the begining of the game, you find that you have a second rivial who's the opposite gender of your first rival. (so if you chose a boy for your character and a boy for your first rivial, the second rivial would be a girl)
the first rival should be the traditional rivial that sports an arrogant additude during most of the game, but ends up seeing you as an equal. Though you still share a rivialy, abit a friendlier one. He/She chooses the starter that is stronger then yours.
the second one is a bit more meek. He/She kind of resembles Yellow from the pokemon manga, and is kind of like Wally or May. He/She looks up to you, an as you help him/her, she becomes a stronger rivial. He/She chooses the last starter remaining.
on a route near the begining of the game, you meet a lass type trainer who has a different sprite from other lasses. However, after overcoming a certain obstaicle later on and becoming famous, she starts to follow you and claim to be a fangirl of yours. She also gains the first form of your starter pokemon.
There may also be a dark rivial, much like GSC's rivial that later joins the evil team or something, but later repents his ways and helps you defeat it.
that's what I'd want to see, but chances are that it won't be that complicated...