A year ago, it was still the case that only Novak routinely beat Roger.
Now, of course he's getting older, but age alone shouldn't make him deteriorate so fast. *ASSUMING* that he is and stays fully healthy (not a given), I would think that he would be more likely than not to make it back into the top 8.
Seeding makes some difference, but only up to a point. Fed would be looking to routinely make at least quarterfinals in his tournaments. It is really only Novak, Andy and (if healthy) Rafa that he probably would like to avoid. The chance of meeting them before the quarters is the same whether he is ranked 9-16 or 16+, so it may not matter so much. (I may regret saying this if he keeps drawing Novak at the earliest opportunity ...)
AO draw could matter a lot in terms of how quickly he starts moving up the ranking, but it doesn't seem realistic to expect him to defend his SF points. Brisbane F + AO SF makes 870 points. If he dropped all of them, he'd be on 2130 - 870 = 1260 points, which on current rankings would drop him out of the top 32. Hopefully he won't drop quite that much, but even with a AO QF he'd be only on 1620, probably mid-20s in the rankings.
Then he has nothing to defend until the clay-court season, in which he's only got 270 points.
He could choose to play Dubai, and will have chances to get some serious points at IW/Miami (though perhaps it is doubtful that he'd play both). If he's going to have a chance to make it back to the top 8, then he should be able to at least make a few Masters semis, and getting back up into the top 16 by the French Open.
If he does achieve that, then he'll be in a good position until the rest of the year - assuming he manages to defend at least his Wimbledon SF. But here the SW19 seeding formula ought to help him, with last year's grass-court results and the previous year's final he should have a chance to be seeded top 8, and he'll still be a force to be reckoned with there.
So I don't see a problem with having him back in London at least one more year, *assuming* that he is and stays healthy. (Here's hoping ...)