I'm not a huge fan of Wallace as champion in Emerald, but it's not a major gripe. Throughout the game, since the story isn't much changed from the original in RS, Steven is still a bigger presence than Wallace. For Emerald, why couldn't they swap Steven's encounters with Wallace to give him a better preface? I guess it was kept the way it was in RS to make it surprising when you find out Wallace is the new Champion, but the fact that Steven still shows up more and helps the player more than Wallace does just further cements his reasons for why I think he makes a great Champion. Wallace, on the other hand, is only helpful in the climax of the story, and even then, he asks you where you should go and gives you completely bogus areas and when you pick them, he says they're not the places to go! Why bother asking me then? You clearly already know where we need to go, Wallace! I'm also bothered that his gym gets replaced by Juan, someone we barely know and hardly get any context for other than that he was Wallace's mentor. He's just out of no where and doesn't really stand out.
I will say, however, that I find Wallace more difficult to defeat as a Champion, so I'll give him credit there! I feel like no matter what starter you choose and how your team ends up, you're never truly ready for him. I always tend to have trouble with him no matter what. Nothing too difficult, but a fair challenge.
Edit: ...I said it wasn't a major gripe, but I sure did say a lot...lol