Here's just a suggestion from a fan of the game.
I think there should be a combination of mercs that are available and mercs that you can choose to join the mission. Even having the choice to pick one merc team is huge -- especially since you can't freely allocate experience between your mercs. Basically, this allows the players to essentially customize their gameplay style.
* Merc 1
* Merc 2
* Whoever you want
This allows you to
1. Essentially build up your favorite merc. (Since you use them more, they gain more experience)
2. Choose a merc team that works with the other available mercs.
Maybe you can't choose the same merc team two times in a row (this forces the player to have more than one favorite.
I think this would help with replayability as players choose to beef up X team in a game. Maybe the next time you decide to beef up snipers, instead of artillery, or marines, etc.