I was thinking that perhaps reactor cores could eventually be installed as tertiary subsystems. That way, newer reactor technology could be installed into existing fighters partway through a campaign. Note, that this feature may not necessarily be compatible with the existing cannon for Freespace, but some non-freespace campaigns could benefit from this ability. This would unfortunately mean waiting for the implimentation of Tertiaries. Of course, the other way to do this would be to introduce a completely new category... upgradable subsystems. Things like Reactors and Shield generators, or pretty much any citical part of a fighter that could be upgraded during a campaign, could be incorporated in this.