Nice to see you are still around CJ. I haven't tried using the REGS version of Nernies expansion but I will see how it works with ICAIO.
Like Teabag I use Inconsequential NPCs but not Interesting NPCs. The only problem I have is with the patch for Cutting Room Floor; it's fairly old and outdated. The CRF changes in the part of the mod that includes the Thalmor Headquarters aren't always compatible with the Inconsequential NPC changes there.
I use two Arthmoor mods that provide expansions of existing places, Soljund's Sinkhole and Whistling Mine. They add some immersion, there don't seem to be other mods that change these, and these sites are ones that ICAIO isn't planning to change.
I haven't tested whether Understone Keep Fountains and Rubble, Better City Entrances, or Better Docks are compatible with ICAIO but I expect there would be some compatibilty problems with these.
I use the Location and New Location mods in REGS except for the Sky Haven Temple mod; I instead use a different Sky Haven temple mod that was previously in REGS. Like Teabag and others I'm not using the Thirteen Oranges mods.
As I mentioned in my last post, I use Inns and Taverns. I also use Frostfall, and in the northern parts of Skyrim it isn't possible to walk very far without having to stop and build a campfire. The stops are frequent enough that walking gets quite tedious (and unrealistically often). Having a few Inns and Taverns in these areas seems more realistic and lore friendly than expecting everyone to run or use a horse, and even then spend a lot of realtime in the game managing campfires. When the modular version of Inns and Taverns comes out I might remove more of the sites in warmer areas. Even with these northern Inns and Taverns it's is still necessary to stop fairly often and build a compfire when using Frostfall, but it seems a little more realistic to me.
I use the STEP mods in the REGS Enchancement section plus Better Fast Travel, which adds some additional locations for fast travel.
Most of the need for compatibility patches with REGS seem to be for lighting mods. STEP favors Relighting Skyrim and there aren't many existing patches for this with the mods in REGS. There are a few more for ELFX Exteriors (used in SR:LE), but there is still a need for lighting patches.