He's a better bet than Reigns is full stop... but the things that make his unique are what will hold him down in the end.
Not Being American - WWE has NEVER had a truly non US poster boy for the company, face or heel for more than a week or two, only Sheik really qualifies even then. They've resisted when solid candidates have been there like Andre (he never got "the ball" only to be an attraction), The British Bulldog (could have done it in 92 or 96, and more recently Barrett (Nexus time he could have been immense if given that title...) they just don't want to do it and Sheamus being Irish isn't going to help... perhaps he has a little more chance than those others as there is such a large Irish fanbase in the US... but if he got it before a Brit for example, the backlash over here would be big and might damage business.
His Look - The ginger, pasty look is totally at odds with what WWE wants to go with for it's top guy... yet he has constantly been told don't change it. While I hate the idea of shaved heads everywhere, darker hair and a bit of a tan (Sheamus goes Hollywood or becomes a Horseman for example) would do a lot for him, perhaps even shaving the beard down to a more managable darker goatee or going clean shaven, he's not an ugly guy and Bryan and Bray kinda have the "wild" look cornered.
His lack of real promo skills - He can talk if you can call his affable "oirish fella" schtick talking but it's not really good at projecting malice or making him sound fearsome... Finlay sounded like a nasty piece of work, Sheamus doesn't... BNB can sound condescending and have a fun streak at the same time... Sheamus can't.
But there are a couple of reasons it might work.
Size - With the exception of Barrett who has inevitably gotten smaller since his injury, Sheamus is pretty much the template for WWE, tall and while muscular, there is still room for some work. Perhaps all the Trips workout buddy jibes stopped him working as hard on his body, either through choice or being told... with a bit of work, he could get back to looking like a beast... then he at least, with a Heyman style manager and less talking from himself get into "that zone".
Popularity backstage - He's a well liked guy, he has long had to put up with favoritism snide talk from the IWC for his friendship with Trips and Regal. Arguably he did carry some bags etc for Trips, but that's something Trips did himself and both guys quickly carved their own niches... but he's not got any major heat with people, is known to be safe and "a good hand"... they can trust him with anyone in the roster to have a good match and not hurt them.
Rumors - We all know that he was in Ireland last year while a certain movie was being filmed... and he even said on twitter he couldn't confirm he isn't in that movie... Yep it's Star Wars and IF he is involved in some way, perhaps as a reincarnated Vader or his "replacement" in terms of stunt work, like Dave Prowse was in the originals then that suddenly puts Batista's role as Drax into the shade slightly... While it's likely not true...if he IS Darth Vader... then WWE have Darth Vader on their roster, sans suit of course but that rep could help with the Sheamus goes Hollywood sort of stuff I mentioned earlier.... It WILL be the biggest movie of all time after all.