I am not sure what the advantage of voxel terrain would be.
Better terrain destruction. But since voxels are
far more resource-hungry than polygons, and OpenXcom already tends to struggle with moderately large and especially multi-level maps, it's doubtful it would be a net benefit.
Especially since you'd be essentially writing your own engine (or engine extension) for it, part-time, with another game in the wings, a day job, etc. Probably would end up as some sort of X-Minecraft in the end.
Surely xcom3 style destruction is good enough?
Depends on who you ask. But since most commercial X-Com clones have never gotten even that back, I'd be on board with it.
For sprites, I plan to get replacement from asset store texture packs (walls)
Have you looked at just
how many of these there are in XCF?
daz studio (humans)
3D? Ugh.
pixel artists from r/gamedevclassifieds (aliens).
You're intending to pay them? For full sprite collections, including Ufopedia? For at least a couple dozen aliens?
Please tell me more, which two core arcs...
The OG alien arc, and the sequel TFTD arc. Both use (mostly) OG assets, lore and concepts right now, and I imagine the specific designs and names (Sectoids, Mutons, Gillmen, Tasoths, etc) are still copyrighted. Even if not, or not fully, nuCom has been pretty successful and 2K has a lot more money and lawyers than you.
...and who do I get permission from for the rest of the arcs? Solarius Scorch?
He'd be where you start, yes. But some of his work is building on existing IP to a degree that won't fly in a commercial game (UAC being the most obvious example). You two would need to sit down and examine the whole thing pretty thoroughly.
...fill the spot with 1. operative 2. armor and equipment. Everything drag and drop. No more frantic clicking to change everyone's armor for a different mission.
How is drag and drop
better? In OXCE, armour can be changed in bulk with right-clicking. Unless you make armour a part of the equipment template, I don't see how you can improve on that. Maybe I'm just dense.