Okay, so how to prioritize needs?
First, let's look at the 2015 free agents by position:
RB - Mathews, R. Brown
WR - Royal, Ajirotutu
C - Hardwick (likely to retire), Legursky, Ohrnberger, Robinson (RFA)
G - Clary, Baca (ERFA)
LT - Dunlap, Smith
DE - Mathews
OLB - Freeney
ILB - Gachkar
CB - Flowers, Wright
S - Gilchrist
P - McBriar
And let's look ahead to 2016 free agents:
QB - Rivers, Clemens
FB - Johnson
WR - Floyd, Inman (ERFA)
TE - Gates, Green, Phillips
G - Rinehart, Troutman
LT - Wiggins (RFA)
DE - Liuget, Reyes, Square (RFA), Scafe (ERFA)
OLB - Ingram, Johnson, Law (ERFA)
ILB - Walker
S - Weddle, Addae (RFA)
First off, it seems likely that the team will want to keep Rivers and Weddle beyond next season, in which case they should sign them to extensions this offseason. Rivers' cap number in 2015 is scheduled to be $17.4M; Weddle's will be $10.1M. If they are extended, both of those numbers can probably be lowered. That, in turn, could help the Chargers to make a short term push while those two key players are still playing well. It will be much harder to win anything if they spend too much time rebuilding and those two can no longer effectively lead the offense and defense.
Next, there has been speculation about the team releasing Gates, whose 2015 cap number is $9.7M. I don't see that happening. Unless he wants to retire, I expect him back. Like Rivers and Weddle, he could potentially be extended/restructured. But he is already 34, so I'm not sure that makes sense, unless the extension is for years he and the team don't expect him to play out and they do it simply to lower his cap number in 2015.
Beyond those players, I don't see a need to worry about next year's free agents.
I agree with Gunz on the OL moves: resign Dunlap, move Fluker inside, move Watt back to G, use a 1st or 2nd rounder on C/RT and sign a free agent for the other. I actually wouldn't mind if they signed free agents for both positions, depending on who is available, leaving both the 1st and 2nd rounders to be applied on defense.
Beyond that, they should probably use the draft and/or free agency to acquire at least 1-2 more OL. Legursky and Ohrnberger are both UFAs, and both graded out terribly this season (per PFF). Robinson is a RFA but also graded out badly. The team should consider letting all of them walk, if they can improve the depth without them. And Clary has been awful for a long time. He should also be let go.
At RB, it's hard for me to see the team going into next season with just Oliver, Woodhead, and Donald Brown. Gunz suggested complementing them with a 4th/5th round pick, but I think it is pretty unlikely to get someone there that moves ahead of Oliver and Woodhead. I would prefer that the team sign an inexpensive veteran instead. That could be Mathews, depending on how the market values him, but I suspect the team will want to let him go and move in a different direction. This coaching staff never seemed to trust him even when he was healthy.
Oliver showed flashes, but enough to make him the lead back? I am skeptical. As I said at the time, Brown was a terrible signing; despite McCoy's praise of him, he graded out terribly this year. I'd prefer to see an upgrade on Mathews via free agency, making Oliver a change of pace and depth player behind the new RB and Woodhead. Unfortunately, they probably won't release D. Brown, since they would then eat $2M+ in dead money; I think they will release him after 2015 instead, when the dead money is half as much and he is scheduled to earn a $500K roster bonus. But I would like to think he won't play much, if at all, unless due to injuries.
At WR, the team needs another playmaker. IMO Allen would be better served as a 1a/1b possession WR complemented by a strong deep threat than as a clear #1. Floyd graded out well, but I think, similar to Allen, he would be better served to move down a spot on the depth chart and play as the #3 rather than the #2. Royal is solid but I think the needed upgrade pushes him out, because I doubt he will sign for good value as a #4 WR. Inman can then move into the #4 spot, which seems fine. Ajirotutu graded out badly on offense this year, but has been really good on special teams. Assuming he will be inexpensive, I'd like to see him resigned.
Moving to defense, PFF graded Mathews as the Chargers' top DL this season. They need to resign him.
Per PFF, Freeney easily graded out as the Chargers' top LB. He is 34, so it seems possible he could consider retiring or even signing with a stronger contender. I'd like to see him resigned to a reasonable contract, if possible.
Gunz recommended keeping Johnson. He played well enough to keep him, but he has a big cap number. $7.5M. Is he worth that? I am skeptical.
Per PFF, Gackhar graded out as the team's 2nd best ILB, not far behind Teo. I'd like to see them resign him if he will sign for reasonable terms. He should be starting over Butler from here on out, were it not for Butler's ridiculous contract.
Per PFF, Flowers graded out as the team's best CB, but Verrett was right behind him despite missing so much time. I think the team needs to resign Flowers and count on that pair as their starting corners for the next 5 years or so. Wright was easily the team's worst CB, and should be allowed to walk. Same with Gilchrist, who also graded out poorly. Addae should be moved into the starting lineup with Weddle. A starting defensive backfield of Flowers, Verrett, Weddle, and Addae will actually be one of the best in the league if they stay healthy.
Unfortunately, all of the defensive moves above amount to just resigning the solid performers and letting go of some lousy performers. Given that the defense wasn't very good this year, that's clearly not enough. I'd like to see the team draft 2 DL and 2 DBs to go along with 2 OL. I'd also like to see an impact DL or LB signed in free agency.
To summarize:
1. Extend Rivers and Weddle.
2. Upgrade RB Mathews via free agency. Let Ronnie Brown walk.
3. Upgrade WR via free agency. Let Royal walk. Keep Ajirotutu.
4. Resign Dunlap. Move Fluker to G. Move Watt back to G as other starter. Sign C and RT starters in free agency. Draft 2 OL.
5. Resign DE Mathews. Draft 1-2 DL, including at least one in first 2 rounds.
6. Resign OLB Freeney if reasonable.
7. Sign veteran starting caliber OLB in free agency. If Freeney doesn't resign, try to keep Johnson as other starter. If Freeney resigns, let Johnson go, unless he will reduce that cap number.
8. Resign Gachkar. Give him a lot of Butler's snaps.
9. Resign Flowers. Let Wright walk. Draft at least one CB.
10. Let Gilchrist walk. Draft a S.
11. Change both OC and DC and possibly some position coaches as well.
I don't expect all of this to happen. I think there is little chance of #11 happening. And I have said here to sign starters at C, RT, RB, WR, and OLB in free agency, in addition to resigning Dunlap, DE Mathews, Freeney, Gachkar, Flowers, and Ajirotutu. Not only does signing 5 starters in free agency not seem to be Telesco's style, it could be challenging to do smartly within cap constraints.
It's going to be tough to truly improve so many areas of deficiency.