Well, adding another Chapter or more to the sector, currently, will be pretty much an overkill.
That is, if the sector is not expanded or the difficulty is increased.
A difficulty increase will lead to a even higher mortatily rate among the MEHREENS. Seriously, I keep losing people so fast, I'm looking at you Tyranids, I can not replenish my losses, but of course, what else can I expect from the GRIMDARKNESS of the far future?
An expanded sector could be a better solution, of course, some pathfinding errors will show up more commonly unless they are solved, like orks and tyranids invading empty systems and staying there.
Another thing I would really like to see is better combat, both ground and space combat, a greater degree of control over the units and orders will be a nice addition.
Of course, I'm not talking about handling every single unit in ground combat, but orders such as a tactical withdraw, orbital support, artillery strikes, support from the guard or other local forces, amongs other options, these options should add bonuses, malus or both to your ground forces.
Say, orbital strikes have a chance of hitting your units depending how close the enemy is, it could save the day or it could doom your forces.
Or an option of requesting reinforcements slaved to percentage of success of the request and only be possible if there is another of your ships in orbit or if your disposition with the governor, Guard or SoBs is high enough, of course, the reinforcements shouldn't arrive immediately, they should take a number of turns depending on their faction, marines with their drop pods should arrive be very quickly while the PDF would take the longest time, of course the response time of the PDF will vary according to the planet, a hive or forge world PDF could respond in the same amount of time as the Guard, but a feudal or death world PDF will take the longest time.