I went to Twitter and asked for some your questions on the 49ers and free agency. Here are my answers:
From @sf_game: Who will be the next [Free Agent] to sign?
While no one will be able to successfully guess who is the next free agent to immediately sign with the 49ers, there is a good chance that the 49ers sign a veteran for depth. Good examples would be S Ed Reed or CB Dunta Robinson who both have positions the 49ers must address. This doesn't guarantee that the veteran player would be an instant starter, but they could easily take a low risk move similar to how the 49ers signed Nnamdi Asomugha and Randy Moss for 2013 and 2012 respectively.
It's not a foregone conclusion that Crabtree will walk after 2014, but I would say yes to answer your question. Trent Baalke has continuously talked about how some players are drafted for the long term where they can develop before being inserted for later use. The 49ers need to brace for any future losses and it would be important that they find a deep WR threat in the draft.
Even though Anquan Boldin signed his contract to stay with the 49ers earlier this month, San Francisco should draft at least two wide receivers. Crabtree is going to be asking for a high price after the 2014 season and the 49ers should indeed be prepared for any situation.
I don't see a replacement for those three positions being signed, but I do expect depth for the offensive line and wide receiver positions to be signed. There are several offensive linemen under contract, but there could be one veteran just to contribute on punts or field goals as an extra blocker.
As for wide receivers, the 49ers will want someone who can help on special teams and offense whether the person is a gunner or returner. The only starter that could be replaced is Phil Dawson if he signs with another team. I believe most of the signings will be for depth and competition, but there aren't expectations that anyone will crack the starting lineup.