Here are my idea pitches. I’ve tried to list them based on how much effort they would(n’t) require compared to making a new map or very big bonus mission, from the little peek behind the hood that modding has allowed me 
Barely have to lift a finger
Contracts mode on Freeform Training, but….
I know it would take two minutes to add contracts mode to Freeform Training, because I forced contracts mode onto that map and made and published contracts for it in H2
But should they make it official, I think it would be beneficial for them to add in the CCTV cameras from the Escalation on that map, as well as the knife from that escalation so there’s at least one melee option for contracts. Ideally they’d place in a couple lethal items around the map, like a screwdriver down with the mechanics, but then they’d actually have to lift a finger and let’s just keep things simple.
Improved Kill Methods in Contracts mode
They’d have to put more than a few minutes work into this sadly, but still overall on lower-effort scale of things from what I can tell. By improved kill methods I mean make it so we aren’t forced to choose between melee or thrown kills… let the player decide! Also, instead of just “Shotgun”, let us hit F to choose “Unsilenced Shotgun”, the game already keeps track of if a silenced weapon was used, may as well make it an option. Would also be great if we could make kill methods mandatory but I won’t push my luck
Improve exits/entrances in some maps (Contracts Mode)
You’ve sort of already admitted this one is a good idea by unrestricting the default start in Mendoza for the ET, and adding in the car exit to the Hawke’s Bay ET
Now just… make them available for contracts that won’t only be played once ever.
You can add to the list the car/gate exit in Mendoza (Isn’t it a bit odd all exits require you to trespass except for Diana dance?), and an exit for Berlin like fixing the manhole exit
Single Player Ghost Mode vs a Timer
All the assets are already made, it’s sad to see them wasting away
Just take what they’ve already got, make it singleplayer and make the existing timer a bit harsher and possibly show up on screen (rather than just the last 20 seconds). I almost already got this working as a mod except for some reason IO has it locked so new targets don’t spawn if there isn’t a second player in the game, so just tweak that little line of code! And also bring over that suit selection menu, which I’m sure they could handle pretty quickly.
Just give it a different name since there are no “Ghosts” involved, something like “Target Practice”, and manage expectations by saying this is just a legacy thing for those two H2 maps that had Ghost Mode. You’ll be very clear and upfront that you’re not expanding this to new maps, and it will be further incentive for new players to buy the H2 Expansion Pass 
Low(ish?) Effort
Add “No Loadout” complication to Contracts Mode
Not as quick and easy to figure out as some of the above suggestions, but it would be great for contracts and the longevity of the game as suddenly it would open a world of new puzzles (And would really make a difference when post-launch content for the game finishes up next year, not to mention make better Featured Contracts each month). Hokkaido’s doors without electronic key hackers, being made to steal a particular weapon for a kill, having to use a gun but only unsilenced ones are present on the map, etc.
Should probably also come with a “No Agency Pickup” complication for best effect.
Special Assignment: Choose Your Own Adventure
You started the beginnings of this with the Pride Escalation. But rather than have it be a weird fever dream broken up into 3 small contracts, make it one large contract tied together with a bit of dialogue and story and with objectives a little more interesting than ‘kill these particular but generic people with this method’. You’ve got all sorts of possible objectives at your fingers: Hack the laptop, pacify and stash someone in a special container, use camera to take photos of secret research, maybe a food shop in Chongqing will pay the player to make 5 people in the rival restaurant sick? Who knows, just make it interesting, and with enough options that it’s reasonably replayable + fun.
It would be a special assignment because there’s no particular need to change the time of day or have any big map changes (nor contracts mode). Add in 2 rival ‘boss’ NPCs to get missions from, and a few NPCs to serve as targets/obstacles for possible objectives you get. You won’t be able to click your fingers and make it overnight, but it’s a good compromise for all the players wanting a bonus mission, let alone a new map, and disliking escalations.
Medium Effort
Sniper version of existing map
Basically do what you did for Colorado - The Vector, although maybe don’t do the exact same thing of having to be fed info from Diana. You can if you want though
Point is it’s a compromise and something you’ve done before in a hiatus while most of your team was working on a new game. And a guy sitting on his ass trying to do a mission from one spot sounds like it would fit in with Sloth to me
Bonus Mission: Hawke’s Bay (Daytime)
I know, bonus missions are probably getting a bit high on the resources scale for what you’re after, but out of all the maps in the trilogy this one is the least effort because 90% of the map would stay untouched. Just chuck some NPCs and beach chairs and umbrella on the beach, the rest will be an overhaul of the NPCs at the house area, refurbishing the existing house, and adding in dialogue and challenges. Also changing the one main light source (Sun) and sky texture, thankfully if me slapping Himmelstein’s sky and light onto Hawke’s Bay can actually look fairly decent, I’m hoping you can do something low effort with your actual tools that this isn’t some Herculean task
Just make sure it gets Contracts mode!
Bonus Mission: Carpathian Mountains (Civilian train, Daytime)
If you pass up a Hawke’s Bay bonus mission then this is probably the next best bet for low effort but impactful. The environment can stay the exact same, just need to change the lighting and sky for it (Your writers can work out why a passenger train is going through the Carpathian Mountains 
). The carriages will need to be changed to not be old and military based, but I saw in the out of bounds area of the map you have empty and unfurnished carriages, blank slates waiting to just be furnished and populated. This leads me to believe it’s a map which wouldn’t be as much work to make a bonus mission out of, compared to the rest of the series. Personally it might be worth making the carriages from scratch so they can be a bit more spacious or like one of those double decker trains, but it doesn’t have to be as long as the train in Untouchable. Just remember to have a bit of fun with the carriages: a bar carriage, a dining carriage, a little spa/sauna one. And make sure it has Contracts mode of course