The Terran-Vasudan war offers a pretty limitless scope for new missions. Only your imagination and how many missions you want before the Shivans arrive will limit you there.
You’re also pretty free to do whatever you want with the first encounter with the Shivans. Only after that do you have to start sticking to the story somewhat.
The PS1 game Colony Wars: Vengeance has hostile aliens arriving and intruding into a an interstellar war. The game has branching paths, so there are two such first encounter missions which both occur in similar style to in Freespace, attacking both parties. They might be useful for thinking about a new first encounter with the Shivans mission. Here’s one:
http://www.youtube.com/watch?v=B2zITr3FFcc&feature=BFa&list=PL6C2AC8FCEE999854 Can’t find an example of the other. In that one, you have to defend a disabled frigate from enemy fighters. After awhile if it has survived, the aliens jump in and destroy your opponents then jump out. Then an alien capital ship jumps in, and unless you can do enough damage to it to make it retreat, it will grapple the frigate and jump out with it!
So that will take you up to mission 5 in the official campaign. There isn’t really anything to take from mission 6 (the asteroid field mission) However, after you beat mission 7, you get told that a separate strike team captured the Shivan shield technology. That is a prime opportunity for a mission.
If you don’t care about having the player control one character and decide to focus on the larger picture, you could stage the battle you get told about where the large decoy convoy is completely eliminated. Since everyone dies there, the player would have to die too. So the objective would either be for the convoy to survive for a set amount of time, or to survive a set amount of attack waves. Or even both. Once that time hits you would end the mission with the debrief that a sufficient fight was put up to convince the Shivans that the shield technology was indeed with the convoy and the Shivans were sufficiently weakened to allow the second stage of the plan, and everyone died to achieve this.
Also, if you don’t care about the player being exclusively terran, you could do a mission where the Vasudans pick up the second leg of the convoy escort. If neither of these apply though, I’d suggest a diversionary strike mission, to draw as many shivans as possible away from the shield shipment. You could even do all three missions.
The final mission of act 1 involves capturing the Taranis, and the player comes in on the final leg of the mission after the Taranis‘ defences have been thinned out.. There is plenty of scope here to flesh this out with one or two missions to flesh out how this was done. You could have something like hitting a Shivan supply convoy to draw the Taranis’ fighters, or playing cat and mouse with the Taranis to draw it’s fighters or intercepting the Taranis’ fighters while they’re on patrol or suckering the Taranis’ fighters with another dummy convoy. Many possibilities.
A further mission could be escorting the Taranis to Tombaugh. I would have mentioned the Lucifer’s attack on Tombaugh too, but the link I posted further up the page, the final mission of that does such a great job of the attack on Tombaugh that I’d leave it alone.
