If you get Carlos Guillen close to what he was, particularly offensive, he can be a big key at that position. He's going to be involved in that mix for sure.
It's not a surprise that Guillen would be a strong candidate for the position if healthy. It's probably more of a surprise that someone thinks Guillen might be healthy. Guillen is coming off micro-fracture surgery on his knee and it's not known whether he'll be ready at the beginning of the season. He is apparently progressing well, but how long can you count on him to stay healthy even if recovers from his knee problem?
The other possibility for second base which has not been discussed much is Danny Worth. The light-hitting Worth batted .255 with a .295 OBP for the Tigers last year, but is the best defender among the keystone competitors. He seems to be a better fit for a utility role though and can play all three infield positions.
Wil Rhymes is the fan favorite, not only because he connects with them through social network sites, but because he performed well during his call-up at the end of the season. He batted .304/.350/.414 in 54 games and fielded his position competently. His minor league record suggests he might have a hard time repeating that success though. He is also not suited to a utility role since he is considered a second baseman only. I did like what I saw from him last year and I'm not going to write him off. He was surprise though and is still considered a long shot.
Sizemore struggled both offensively and defensively last year, but was never fully healthy. He entered the season coming off an ankle injury and later hurt his hip. He batted just .211 in 33 games for the Tigers, but looked better in September. He batted .298/.378/.473 for Triple-A Toledo last season. In 2009, he had a combined .889 OPS for Toledo and Double-A Erie.
If Guillen is healthy, he'll likely get the first crack at second base. I'm very skeptical that hat he can stay off the disabled list for long though. My guess is that Sizemore ends up getting the most time at second base among the four contenders.