Regarding Sherlock Holmes and/or reusing assets from other games...
I'd think the simplest solution would be to buy a royalty free license for stock art or borrow from Champs, at least in the distance/skybox, and then retexture it to better match STO's visual style. Which is not unlike how Thomas has periodically taken photos and then painted them to look like in-game art, like he did for the Enterprises that weren't in game for the Fleet Starbase.
I also think you could accomplish a lot by keeping heavily distinctive stuff in the skybox or at a distance, kinda like Starfleet Academy.
Baker Street might prove tricky but the banks of the River Thames and then some stock alleys might be more doable.
I realize this might be controversial but this is an area where rail shooter travel by carriage could allow for a lot of cheating if you wanted something visually impressive without too many load screens and on the cheap. You'd just need control of where the player is in relation to some things.
Malfunctioning holodeck adventure zone/battlezone. You wouldn't need a full kit of assets for any particular program. You could have a crazy skyline like we see in Doctor Who's "The Wedding of River Song" and partial sectors by area.
It might scracth a lot of different itches at once but you could get by with a lot of partial settings rather than any one complete one by having mismatching buildings and assets, deliberately, and flashing holodeck grid effects in place of even having to do full textures.
Victorian highwaymen, robots, and dinosaurs could make up a single mob group. Yes, yes, a lot of art but none of it needs to be a complete matching set, which would let you cheat in all kinds of different directions and plug in gaps with Bajoran assets, Cardassian assets, Klingon assets, etc. In that case, you could have a lot of novelty but get by with elements being incomplete because the zone is, deliberately, chaotic and mishmashing existing assets with hints of new ones.
No, it would have to be a pretty substantial thing. Think of all of the assets that would have to be made for it. If we're investing that much into something, it would have to be a big deal. Something like an event, or a big part of a season release.
Doing other Holodeck adventures would be equally expensive. Think about Dixon Hill, or Sherlock Holmes. There is almost nothing in the game that could be reused in such a mission. That's the problem.
Season 10: Photonic Rebellion
Where the Human Adventure is Just Beginning! <=============================
Exploring STO Since Pavement.
Tier 2 Constitution for Fleet Refit!