Well for starters I don't think David Boreanaz and James Marsters aging would be that big of a problem. I really don't. Like [MENTION=8404]Blaze[/MENTION] said, I'd be more than able to suspend my disbelief on that one. And let's not forget either, if something like this were to magically come together, both of them would hit the gym like crazy to look as close to the part as possible.
However, that being said, I'm not entirely sure a new series with the same general cast would work. Almost all the principals are engaged elsewhere. A movie? Maybe (and that's a very slim maybe), but it would be tough. I don't think it works without just about everyone. I mean, you'd need Buffy, Dawn, Angel, Spike, Willow, Xander, Giles and probably Faith minimum. That's a big task to get everyone, you know?