The last time this came up, badp mentioned something that I think still holds true:
One tag per game. Two tags for a game is not how SE rolls.
Given that on one of Jeff Atwood's more recent visits, he specifically mentioned overtagging as a problem, I think any solution that involves more tags for a single game is not likely to be acceptable even if we feel it is a workable solution.
We already know that the 25-character limit is fixed indefinitely. Unless the SE folks are willing to spend dev time on a solution like the one LessPop links to on meta.SO, I don't think we have an alternative to series-gamena.
I do think that a two-tag approach would cause a bit of confusion: the tag wiki for the series, in particular, would require more explanation, and then we would probably be answering questions on meta as to why we need to use assassins-creed but not madden or rock-band. (There may be fringe cases where a game name is used by multiple series – explaining that revelations applies to both Assassin's Creed: Revelations and, say, a future Ratchet & Clank: Revelations could be a big issue – but I don't think that's as important unless we can identify one that would apply now.) Unfortunately, there isn't a good way to estimate confusion about multiple tags vs. confusion with abbreviated tags. If there were, we'd have something more than opinions to work with, and maybe that would influence the SE folks.
I think a consistent policy is easier for people to understand, even if it means we have some tags that are not intuitive. I would vote to keep the one-tag-per-title rule at the expense of the longer game names.