Okay, so we've got the following potential plotlines coming up in the future:
Making up with Tempo
The Siege of Nezel
The Bedein Encounter
The Gunship Arc
I'm Not Your Buddy, Friend (Jardin/Male Encounter)
Making up with Tempo has to happen at this point. Leaving her hanging for too long doesn't feel right with how he's been characterised and it wouldn't feel right for Alex to maintain a grudge given the full contextualisation of just how badly she could have screwed him over. It'd probably have to be the last of the 'quiet' arcs to occur before the following.
The Siege of Nezel and the Bedein Encounter should probably occur in tandem, with the fight for the system and boarding action being executed in sequence. Ideally the climax of both plotlines should dovetail for an exciting double crescendo of action and revelation.
The Gunship Arc should be the next 'quiet' plotline to be executed after the battle of Nezel, assuming it's successful. I'd argue that Alex should probably take a more supportive role in this one, whilst Talon and Spiral take the fore on this. Alex's opinion can be easily dismissed as male, alien, and outsider by the Tenoin clan elders. They can't just as easily dismiss a pair of ace pilots who are set on making the gunship a workable platform. There'd probably be at least a handful of gunships in mothballs from the prototyping and final design stages, so there could potentially be a wargame or live fire test to feel out its viability. Politics and intrigue potentially abound, both internal and external.
Jardin meeting with his opposite number is something that's been on a number of people's minds. Could happen at any time, but I expect their conversation to be interesting and potentially enlightening. Cultural exchange is always as fun to write as it is to read.
Beyond that, I'd suggest the following:
Each character gets an arc where they stand in the spotlight. Alex is arguably the viewpoint character, and I'm not arguing that we change that, but there should be sections of the narrative where the other characters are driving things forward. A few ideas I've had in mind in no particular order:
Barsam Bedtime stories: A discussion of Barsam theology, Alex's feelings thereof, and perhaps some insight as to how humanity in general might react to a few of the troubadour-clerics paying them a visit sometime.
Encounters with various Union members, Arekka, Pol, Neridi, perhaps even the Mannadi, don't really have a plot in mind but one might shake out.
Beryl gets called in by her caste elders to relay and distribute her information about humanity. Comes under some scrutiny and potential fire for perhaps getting too close to her research subject.
Fireblade has to face the last of her internal demons. Perhaps a fellow survivor of the Seren occupation? Another Teidar who challenges her to a duel? Kind of vague on this.
Tempo regaining her honour. Best I could think of was that some shadowy group tries to abduct Alex and Tempo saves his butt by being the glorious lying spook that she's purported to be. Also Jedi. There must be Jedi references.
The Long Road: This one's been bouncing in my head, but since New Frontiers is already heavily referenced, perhaps a few or even just one of the lost diral survived to this day? They would be pushing into their old age but it's more than plausible that if any had survived all this time then they'd push for meeting Alex. Probably best to let it sit until New Frontiers is further along though.
Imperial Imperative: Alex and company meeting with the Emperor. Has to happen sooner or later.
Stillstorms Glorious Revenge: Exactly what it says on the tin. Another backburner project, but one that I believe should have Stillstorm planning and then leading and offensive into Shell territory Sunfall style, except better and with a lot more dead shells. Could potentially be a culmination of all the prior arcs, with the gunship program in full swing paying dividends, the Bedein study paying off and so on.
The Journey Home: With the war well in hand, the situation stabilised, Jardin heads home with a delegation of Loroi to formalise diplomatic contact between humanity and the Loroi union. Not necessarily the end of the story if people want to keep writing for it at this point, but potentially the beginning of a whole new set of shenanigans.