^This^… Coherent packs… You can never have enough good solid environments on the marketplace imo - from all game genres or eras of history. Overall, environments are probably better for sales too (versus character packs or props or some of the other options maybe, which can be quite a bit riskier for sales 
). However a few important caveats about environments… Always add proper collision / double-sided meshes / double-side materials as necessary. Here’s why…
Think about how buyers or end-users might actually use your asset packs. Ask, how could this be used in practice… As too many packs are overly self-contained / individual / fixed. When that happens, they’re often way too difficult to integrate into anything else without a whole lot of re-work for buyers.
For example… Creators too often assume that all packs will only be viewable from ground level (z=0). But when viewed from above or from the outside exterior or map limits, they totally break down (the illusion is 100% gone).
Which often ties in with creators over-optimizing packs to help inflate frame-rates. This isn’t helpful. Take packs from the free marketplace this month: Cambodia / Paris / Cave-Ruins. None of them work particularly well from any of the perspectives mentioned above (think top / exterior / outside map limits).
Ongoing marketplace issues… Think packs with bad collision, where creators cheat with doorways that can’t be entered by standard sized pawns and so on… Or modular packs that are anything but… Or modular packs that are way too modular (buildings with 1000’s & 1000’s of pieces - not always helpful especially when its just a ruin / deserted / rundown building not a palace or key location).
Counter-intuitive maybe, but it’d be nice to see packs that are shipped with per-poly / complex-as-simple collision enabled by default. Now that Fab is adding more options, open to seeing packs sold in multiple forms or sold with customization side-options, especially for environments that can be used in games with flight-sim mechanics PLUS ground action. Such as packs placed in valleys, that can be viewed from above but can played from above as well on ground level too…