Make it so that you can’t complete AV without the other faction getting in your way.
I propse an NPC spawns in each base and walks towards the enemy base along the road. Each of these starts the march once certain objectives on the map are met. Perhaps it’s first faction to kill a certain NPC on the map or first faction to 100 kills. Or something more logical that fits the AV narrative. I’m sure someone here can think of a good trigger that encourages PvP.
The NPC must make it to the enemy stronghold in order for it to be feasible to take the enemy leader. This forces you to defend the NPC in a large group. If they die they will respawn but it will set you back. This preserves the samey gameplay of zerg vs zerg from 1.5. This NPC would not be that helpful in combat as to make it unfair. But it would require healers to heal him/her constantly. And players with CC to control those who wish to snipe this leader. So much strategy and meta could go into this. For one thing you would need to kill other players and also CC the players who are the biggest threat to the NPC.
Of course all other interesting features and summons can still exist. While the two escorts are happening, these summons and war effort quests would really aid the escorting of the NPC and would actually be relevant in pushing to the enemy base.
Capturing towers would still be releveant, etc. The key here is that you can make it the same game by tuning the objectives. No it’s not exact 1:1 vanilla in terms of mechanics. But it will be in spirit.