I think he has a better shot of being World Champion if he heads to SmackDown via another Superstar Shakeup. When you take into account the fact that Jinder Mahal of all people was WWE Champion for 6 months, I think that the "there's no possible way" argument for just about any man on the WWE roster can be made anymore as Jinder is one of the least talented men on the rosters. On Raw, Vince was in full hoss mode for virtually all of 2017 with guys like Goldberg, Lesnar, Reigns, Strowman and Joe all vying for or becoming Universal Champion and that doesn't look like it's changing anytime soon. It's all but guaranteed that Lesnar keeps the title until WrestleMania where he'll drop it to Roman Reigns and I could easily see Reigns finally getting that long title run; in my opinion, he wasn't ready for his WWE Championship push and while some fans will continue to detest him no matter what, it's hard to argue that he isn't ready now. He's a constant source of fan interest and investment, he's continuously having strong matches and has had too many big moments already so if Reigns takes the title from Lesnar in April, expect him to carry it for most of, if not the rest of, 2018.
If Miz went to SmackDown, his chances of becoming WWE Champion once again go up. While SmackDown Live is often the better show, not as often as it once was but it's still usually better, and even though it's the current home of the original World Championship of WWE, it's obvious that Vince still views it and treats it like it's the inferior brand.
Miz isn't bigger than the WWE Championship, Miz is, and in my opinion always was, highly entertaining as a heel with a great work ethic and has been vastly underrated by internet fans over the years, but he's not bigger than the title. I think he's at a point in his career where he could have been but after he lost the WWE Championship, Miz spent years pretty much spinning his wheels in lame duck feuds and one miserably depressing Intercontinental Championship run after another; it's only been within the past few years that we've seen a resurrection of the Miz, he's been great for the Intercontinental Championship and has, overall, been doing the best work of his career but those few years aren't enough to make him bigger than the title and Miz would gain a lot, potentially, by becoming champion again. Also, you have to take into account that despite the good entertainment value he's brought and the fact that he has also spent these same few years as a great Intercontinental Champion, he's ultimately been used to put over, or to at least be the guy who gets beaten up by, the bigger names and/or the guys Vince has big dreams for like Cena, Reigns, Lesnar, Strowman, Joe, etc.
With that being said, I doubt that Miz will be champion in 2018. By the lay of the land right now, it looks very much like it'll be Styles vs. Cena at WrestleMania whereupon Cena wins his 17th World Championship. I could see Miz reigniting his feud with Cena, but it'll be abysmally one sided, as it always has been and as it was during WrestleMania season last year during the mixed tag match build up. Besides that, you know Miz will be taking time off to spend time with Maryse after she gives birth to their daughter.
I see Miz wrestling only for another couple of years, I don't expect him to hang around long past 40, so I could see him getting sort of a brief, farewell run as champion but I ultimately expect Miz to stay in the Intercontinental Championship picture for most of the rest of his career. I think they'll have him win the title a total of 10 times, breaking Jericho's record of 9, and, even before he reaches #10, he'll have held the title for more combined days than any other champ in history. So yeah, I think Miz's legacy will mostly revolve around the Intercontinental Championship.
"What Do I Know Of Cultured Ways, The Gilt, The Craft And The Lie?
I, Who Was Born In A Naked Land And Bred In The Open Sky.
The Subtle Tongue, The Sophist Guile, They Fail When The Broadswords Sing.
Rush In And Die Dogs - I Was A Man Before I Was King."
Conan Of Cimmeria