First of all, a word of encouragement to the devs who are doing an amazing job at polishing the game and making everything they can to improve performance. This new build really feels like a step in the good direction and though we are still not quite at the same level of smoothness Insurgency2 players like myself are used to, let's keep in mind that these are two different engines with their specificities and requirements.
That being said, I feel like content-wise there won't be much change or additions between now and release. There are however a couple of things I would like to leave here in hope they get the chance to read it and maybe consider for post-release content :
More military inspired gear for Insurgents customization ; as of right now, it's basically tracksuits vs camos. Though I understand you also have to be wary about enemy identification, there is probably some wiggle room to add more cargo pants, jackets, some different camo patterns to give them more of a Chechnyan or Afghan rebel vibe
Though I am not necessarily in favor of an incontrolled growth spurt of weapons, I still feel some classes deserve a bit more love. Gunners need at least another option to complement the existing one : with the 7.62 PKM for Insurgents and 5.56 SAW for Security, this leaves room for say a 5.56 Galil or RPK variant on the Insurgent side and a 7.62 M240 or M60 for Security. The Advisory on the Insurgent side also needs some more variety. Maybe old timey weapon such as a PPSH, STG 44, Sten/Sterling and such could come back to give this class more fun options.
Generally speaking, I preferred the way weapons were allocated in the previous Insurgency game, with the AK family to Insurgents and M16, M4 and such to the Security. This made both sides equally attractive with their own sets of weapons to choose from, while Sandstorm has given Security an access to both its own and Insurgents' arsenal, making the latter a lot less interesting.EDIT : this part is now irrelevant as armories have been rebalanced!
New and more varied maps. Night maps will hopfeully make a comeback down the road. Given the theme, I understand the focus on urban warfare, but insurgency wars are fought in various settings such as dense forests or snowy mountains. The Source Engine had its limitations but when you see what Squad, Post Scriptum or even Rising Storm 2 (with an older UE version!!!) have done, this gives room to a lot more fun options in terms of future maps instead of just deserts and cities...
These are just some of the ideas I'm throwing out there, feel free to add, comment, reject or validate what you want!