I can't see it being as anything more then a non-wrestling role. He's noted in several interviews such as with Bill Simmons of ESPN his desire to honor his word to The Undertaker, whom he is said to respect more then anyone within the company.
As for a return as Nexus leader, or a manager type role, the biggest reason HBK retired was to be a full time father. He already was working on what was known as the "HBK schedule", where he only worked Raw's, and PPV's. So if he walked away from that, why would he return as a manager or stable leader?
The most likely option I see happening, at least in the forseeable future, is for HBK to headline the class of 2011 Hall of Fame at Wrestlemania. WWE is short at this time on big name HOF's, and HBK would certainly fit the bill. You could possibly see him involved in a short term angle around that time, but it's not likely to involve any wrestling. The only other thing I could speculate on, and it would be a long shot, is this....
HBK was the reason Nexus buried the Undertaker. Yes, I know that it sounds ludicrous and like a long shot, but Undertaker did retire HBK after defeating him at Wrestlemania. This would set up the very unlikely scenario that HBK is the secret Nexus leader, and the reason they've kept the reason under such close wraps. What would produce bigger shock value then that?
I think its the biggest longshot of longshots, but its something to simply consider.Everyone wants to see an HBK return and a great payoff to the Nexus storyline, and I can't think of much better then that. But the truth of the matter is, even if HBK is going to return for something, its not going to be to a large role that so many would like to see. He's simply given his all to the business, and I think this tweet is being too read into.