I have an offer for alliance system.
1 create coalition system and war list. Change color of you friend and war alliance on the map. It's really help with attack and coordination.
2 create some additional bonus in alliance. May be you can create alliance base (or may be you can create it on map and alliance should take away it from NPC). All in alliance should protect it, upgrade it and base give some bonus (attack, def, oil, metal, etc). Base should be on the map for bonus. Another alliance can crush it and it's give some presents (for example war alliance should stay near your base 30 min, if you are not in War list you can't crush base or all can crush your base at some random time - for example next attack to your base will be at 18:00).
3 change FOB, at this moment all FOBs can kill with 100 oil, FOB should give protect (at this moment it's free metal)... may be also, you can change amounts of wave add 2-3 wave (first after 30 sec, next 45 sec, next 60 sec, it's add reason for build it).
It's not finish offer for alliance system. All should be discus and find better offer for alliance, but ialliance should be change... at this moment no reason for low lvl players be in alliance (37- lvl). Play on 47- hard but still you can play without alliance.