Eccleshare himself has done little to discourage such speculation. "I have made no secret of the fact that I wanted to get back into adland, it's true, he says. "I had a huge learning experience but I had underestimated the culture change I would face. McKinsey is a much more serious-minded place than any agency I've worked in. In a sense it lacks the huge variety of types of people and problems that you get in the best agencies."
He's also clear about one aspect he missed during his two years away.
"Don't underestimate the creative side, he says with hindsight. "It is the creative department that gives agencies their energy and personality."
But Eccleshare is insistent that the experience of integrating brand and business strategy will be crucial to his new role as the chairman and chief executive of the Y&R/Wunderman Alliance in Europe, the Middle East and Africa.
"He's always been very interested in broader business issues and the expanded communications area, Miles Colebrook, the group president of J. Walter Thompson International, who first hired Eccleshare as a trainee, confirms.
Eccleshare spent 18 years at JWT, where he progressed to become the network's worldwide director of strategic planning. He then moved to APL, where, as the chief executive and then chairman, he oversaw some turbulent years, which included the loss of the £40 million Rover account. He was among the senior redundancies made at APL after its merger with Lowe Howard-Spink in 1999, which saw Lowe's Paul Hammersley become chief executive.
Now Eccleshare is charged with overseeing the alliance of Y&R Inc's two network subsidiaries, Young & Rubicam and Wunderman, into a single management structure across Europe, the Middle East and Africa. The strategic move is intended to deliver the integrated, media-neutral solutions so often promised by agencies.
Although it may be a tricky brief to marry the two networks, it's perhaps an inevitable step, with some industry observers surprised it has not happened sooner. The integration will be driven from the top of the two networks' management structure, and Eccleshare will report to both Y&R Inc's chairman, Mike Dolan, and the worldwide Wunderman chairman, Daniel Morel. It's a step that Y&R has already taken in the Asia-Pacific region and parts of Latin America.
Eccleshare's EMEA brief is a newly created role, so he has no way of learning from a predecessor's experiences. Still, he's picked up the gauntlet with gusto. "I think Mike Dolan has a very clear vision of what he wants to do with the Y&R group, which absolutely fits with what I've been hearing my clients say they want from agencies, he says.
However, many observers contend that allying Wunderman and Y&R will prove a challenging task. Despite some strong performers, Y&R's European offices have enjoyed mixed fortunes in recent times. The senior management of the top-ranking Swiss shop Advico Young & Rubicam announced their mass departure in March in perhaps the highest-profile blow to the network.
And the role brings its own issues of turf ownership. Insiders cite the friction that already exists between Y&R's European management and their senior American counterparts and warn that Eccleshare could find himself caught in the crossfire. "The real challenge William will find is that he is walking into a political situation, one says. Eccleshare's advertising background might not help, since it risks breeding resentment at the below-the-line network. "It's a brave move as, by doing it, they're probably pissing off a lot of the Wunderman management, another insider says.
Stuart Pearson, the chief executive of Wunderman Europe, does not agree.
"Y&R is uniquely blessed with two strong brands. We've already implemented the alliance in a number of European markets and it's logical to do it on a European level."
And due to the changing nature of the communications industry, it's perhaps no longer relevant from which discipline one hails. As Colebrook puts it: "William's a bit beyond just an adman."
Adman or more, Eccleshare faces a significant challenge in forging the alliance, the success of which will partly ride on the two differently disciplined networks being able to offer the same quality to clients.
Colebrook responds: "Getting two businesses to work together and hold hands in unison is always tough - but that's the future of the business."
Eccleshare is upbeat. "Part of my job is to build the overall standard of each network. Compared with any other network I've looked at there are real latent strengths in both, he comments.
To do this it's clear Eccleshare will be given a long leash by Dolan, who sees him as a team builder, an experienced practitioner of integrated communications and a client developer whose time at McKinsey has given him unique experience. While a certain amount of resilience will be needed, Eccleshare is looking forward to taking up his challenge.