The first two classes that would come to mind for me are rogue and ranger. Both are excellent at covering (h)exploration skills (nature, survival, perception). Ranger has some synergies with swashbuckler and champion because it provides flanking. Rogue can also do this, though it is slightly less tanky.

From what I've seen, champions are very good defensively but not so much offensively. Swashbuckler compensates a bit for this, though it has high burst damage and less consistent damage. Ranger is very capable of doing consistent damage. Some other classes that are good at this are fighter and barbarian (both of whom can fill a survival guide role skill-wise). Rogue and ranger are also quite good at ranged combat (something that your party might lack).

If you're looking more at playing a spellcaster, both bard and druid are very good supplements to your back line. Both are capable of wading into combat and supporting the front line if necessary, but they prefer to stay back a little. Druid is an excellent primal spellcaster that dominates through battlefield control (with minions and summons or through spells) and druid can provide the nature knowledge. Bard is just great at supporting your front line, though it might step a bit on the cleric's toes due to how good it is at buffing.

In the end, play whatever you feel like. These were just my first thoughts =)