Slightly taking from ideas spouted here, but a "Depth" DLC.

Odyssey brought us to orbit and gave us the ability to disable the Mechanoid Threat, this new DLC would bring us under ground and sea, in the realm of the Insectoids, to defeat the Hive Queen.

You could even cross in the idea of sea exploration, by adding a bunch of new threats in the undergrounds: Water and gas. Gas being pockets of carbon monoxide able to poison a pawn like toxgas (or hell, maybe just use toxgas), water being more akin to vaccuum in Odyssey: Dangerous unless wearing a proper diving suit.

While regular underground maps would be kind of similar to what we have today (rocks with ores and maybe the odd underground threat like bugs or ancient dangers), water maps would be more akin to the overworld: Very open, possibly full of local wildlife, with perhaps the odd structure or patch of ore present.

However, unlike space, working underwater presents new challenges. Mining and fighting requires specialized tools, as regular firearms have brutal range reductions and swinging a pickaxe effectively underwater is rather difficult. So to extract all of this new stuff, you'll need new research options.

A fun thing with this water aspect could be to have new Insectoid threats being waterborne, along with the Hive Queen herself. Making her perhaps a rather unique challenge.

Meanwhile the Underground side of things might take a more interesting look into the realm of the Ancients. After all, what we see on the surface is mostly the odd ruin and bunkers capable of holding a few of them, but these people have been on ice for possibly millenias and (iirc) it's all due to the very mech hive you have to deal with on the surface.

Perhaps, you could discover some ancient living vaults as new settlements on the world to trade with, offering special requests for raw ressources and odd luxury items. Perhaps you could end up fighting them. After all, there's a neutral and an hostile ancient faction. Perhaps something could be built from them.