The only problem I have with your level tigaer is that the Spy objective seems so tacked on. Even though the Merc objective is pretty vague, it fits. They are just guarding something.
In my head I'm imagining a version of Story mode that would make use of vague objectives. Imagine if Story mode wasn't just 1 map, but a pack of maps that are determined by who wins or loses the round. Start with a Neutral map. The idea of this map would be that both sides are trying to figure out who each other are (or that they are fighting for more information). Depending on who wins or loses then goes to a map more personalized for the group that lost (plus maybe geared more towards the losing team). To make the idea simple, we'll say there is only 3 maps in each pack (1 neutral map, 1 Spy centric map and 1 Merc centric map).
Spies lose, Mercs move onto "Spy base" map. Geared more towards Spies because it's their base. So more air ducts, maybe doors that only open to spies (so Mercs have to hack) and generally more places to hide. Something to give the Spies a little bit of a handicap. If the Spies lose then the "Story" ends. Mercs killed off the enemy. If the Spies win then everyone gets transferred to one of many neutral maps.
If Mercs lose neutral map, then Spies move on to "Merc Base" map. Like the "Spy Base", it is more geared towards Mercs. Doorways opening to them, less air ducts, more environmental objects that need to be shut off to help spies (like how you have to hack the glass ceiling in Club House to gain easier access to Garden Room).
Maybe the Spy/Merc base maps are the only maps were Spy/Merc is able to switch weapons/gadgets mid-round. Since it is THEIR base.
Anyway, in the end, the Story mode always ends on 1 of 2 maps, but you have many "neutral" maps that consist of everything other map created by the community or devs.
If someone doesn't want to play Story mode, then there could just be the option to play Objective mode. Kind of the same as what Story mode is right now on CT. Pick a map, pick Objective(s) and play.