The biggest challenge will be cutting through the noise. You know yourself that content is being created and "consumed" (I despise that term) at a rate of knots now with both Social Media and native advertising playing a huge role in peoples browsing behaviour. So now that every single agency is going for "content" as the holy grail of an online marketing plan it's becoming harder for the good agencies to communicate whats actually good and whats utter shit.

A way around this is to fully understand the market within which the business operates. I've said before, content rarely works for B2B in any way, shape or form. In which case, if that's not going to work what is? Surely just getting the site right isn't enough? Depends. As long as thats understood then you won't spend 6 months fighting against a market that you're never going to influence otherwise and a client thats unhappy you're not able to do what they want.

tl;dr too much content. Too much shit content.

Will the challenge change? Perhaps. Google is still at core, an algorithm. We know it looks at engagement factors such as comments on page and some causation/correlation with social media, but it's still limited. If there's some improvement to the algorithm that somehow determines if the content is actually user centric and useful (I know it's bending this way) and if it can determine that "top 10 uses for an old ipod" on a dating website is actually shit then we'll be some way to getting where we need to be.

In terms of your CMS' which ones are you working with? I tend to always point them towards WordPress where possible, in terms of design aspects, if they're on a really old .net solution or something else then the only real recommendation that can be made is for a new design. If thats out of the question, then I wouldn't work with something doomed to fail, personally.