There are many options for the Cubs to fill their void at first base for next season, Below I will breakdown the candidates and their positives/negatives.
Tyler Colvin - While Tyler showed substantial power in his rookie year, he didn't provide much else, with a subpar OBP, and a propensity for strikeouts. I think Colvin is a useful player best suited to the outfield, his bat just isn't good enough to hold down 1st for any length of time.
Aramis Ramirez - Ramirez has been prone to injuries the last few seasons, moving him to 1st might protect him from the wear and tear of the more challenging 3B. Aramis's fielding has dropped off greatly at 3rd also, so moving him to 1st and adding a better fielder at 3rd would help the team defensively. Of course moving A-Ram to 1st opens a hole at 3rd, replacing his offensive production from that spot isn't easy. Adrian Beltre, Jorge Cantu and Ty Wigginton headline the free agent 3B class, and there are options on the trade market as well, but this will cost talent/money. While moving Ramirez to 1st is an option, I don't think it is the best way for the Cubs to go.
Free Agent Options:
Adam Dunn - The positives and negatives of Mr. Dunn have been discussed throughly in other posts. My position is that he is fine investment for the right amount of years (2 & a team option) and dollars.
Lance Berkman - The Big Puma had a pretty rough year, both at the plate, and in terms of injuries. He has stated in the press that he is looking for a full time position, not a platoon role. I don't think this is realistic, unless he returns to his normal production levels. If Dunn gets bid out of the Cubs price range, as I expect him to be, Berkman is my next choice. I think he will be cheap and a good candidate for a bounceback year. However I think a strong platoonmate needs to be signed as well, preferably someone who can backup A-Ram. My choice would be Aubrey Huff.
Victor Martinez - Victor will want to be paid as a hitting catcher, not as a middle of the road 1B. I think combining Soto and Martinez in a Catcher/1B rotation would do wonders to keep them both fresh, and provide injury insurance for Soto. This is one of my favorite options, but he will likely be priced out of the Cubs range.
Aubrey Huff - A lefty with good production last year, he has also played 3B and OF, which would be useful to backup A-Ram & Soriano. I imagine we would have to pay him well to get him away from San Francisco.
Jorge Cantu - He can play 1B & 3B, but he offers power and little else. Probably a good backup option.
Adam LaRoche - He is likely relatively cheap, but he is inconsistent in production.
Derrek Lee - I'm not sure he would come back to the Cubs for a salary commensurate with his performance. In any case his history with the team would make it hard to bench him if he underperforms. I think the ship has sailed on Lee on the northside.
Carlos Pena - I don't see the Cubs springing for a .200 hitter as a starter.
Xavier Nady - He is a platoon option, and his ability to half-play RF is a positive.
Ty Wigginton - His multiposition eligibility would be nice, but I don't think he has the production to start at 1st.
David Ortiz - He has spent much of his career as a DH, playing 1B only sporadically in interleague play. He probably isn't an option for an NL team, but someone may decide to try him at 1st, depending on the cost.
Adrian Gonzalez - The Padres have acknowledged that they are unlikely to sign him, and will probably ask for quite the package of players/prospects to acquire him. He is a fantastic hitter, even more so when you consider his homepark. I think the Cubs would be better served to wait until free agency to sign him instead of moving high level prospects for one season and a chance to resign him.
Albert Pujols - I can't imagine a Cubs/Cards trade for this player.
Billy Butler - A productive hitter with seasons left on his contract, he is probably best suited to DH, but has been the Royals starting 1B for several seasons. He will likely command a good package of players to acquire.
Kila Ka'aihue - Still an unknown quantity, he could be had for less than Butler, but I would be uncomfortable going into a season with Kila as the starter.
Mike Lowell - He is considering retirement, but I think he could be worth a league minimum flyer.
Yonder Alonso - Sitting behind MVP candidate Joey Votto, he is unlikely to break into the bigs at 1st. The Reds would ask for a premium to move him in the division, but he would be worth inquiring about.
Lars Anderson/Anthony Rizzo - These Red Sox prospects have performed well in the minors, but not to the prototypical standards of a power hitting 1B. They are behind Kevin Youkilis on the depth chart, and might be worth trying out.
Matt LaPorta - He hasn't really produced like the prize hitting prospect the Indians thought they were getting in the CC Sabathia trade, but he hasn't been terrible. A change of scenery might help him out, and he isn't likely to cost premium prospects.
I don't think the internal options are a good choice, and many of the trade options will be expensive for the return we get. I think free agency is the way to go this season, particularly as our 1st rounder is protected.