So admittedly, I've already built this and it's fairly solid for drones, i.e.; does what it says on the tin. When I feel like doing something more rythmic I have an MFB Urzwerg Pro, Electro-Harmonix 8 Step Program and a TT-303 hence the lack of any sequencers. For effects I already had a bunch of guitar pedals with plenty of pitch shifting options and rack mount FX so the ADDAC200PI though I realise the drawback of those is a lack of CV. The primary focus of the rack is indeed drone and dark ambient, but I'm also interested in industrial, IDM, acid house and breakcore.

Most of the modules I chose for this I did so because they could do things I couldn't find a cost-effective way to do outside of modular (hence the E350, Morpheus, Warps etc.). A few of the modules here do synergise well and I feel like I have most bases covered including even sampling in a limited sense, but there just seems to be a certain something missing and I don't know what it is; I've considered going for a third row but actually finding it harder to come to a decision on what is most needed.

Here are some modules I've considered:
Serge triple waveshaper
ADDAC 207 quad intuitive quantizer - seems like a logical option for times when I need drones to mesh with other stuff outside of the modular tuning-wise
Mutable Instruments Shelves - a friend of mine once blew my mind with a certain sound when he modulated the frequency of an EQ at a high rate, this would give me the ability to do that on two channels and I could just use it as a conventional EQ & filter alongside it. It'd also make the ALM EQ obsolete though, most likely.
Zvex Instant Lofi Junky - A chorus which you couldn't come across too easily outside of modular. Sure there's also a version of it in pedal format but that doesn't have the CV control!
Expert Sleepers FH-1/2 or Shuttle Control - obviously for MIDI to CV/Gate control, I'm really on the fence about whether either of these would be useful for me. I have a MIDI I/O PCI card so it would mean I could control the modular with software, which is potentially a big deal?
Intellijel Shifty - looks like there are some useful utilities on offer here, I do sometimes find that I wish I had more than the one channel of S&H from the Pittsburgh Mod Tools.
Zlob modular VnIcursal - You know what they say about how you can never have enough VCAs, and I can't get enough of the idea of combined VCAs and mixers. This is like Intellijel quad VCA only even more space-efficient to me, and if I need the features of the quad VCA I'd be keeping that.
2hp LFO - between the Batumi, Kamieniec, Mod Tools etc. I have a few LFOs, but a couple more couldn't hurt and might come in handy when there's 3 rows to work with?
I was thinking in addition to this, maybe a compressor, since modulating the E350 tends to cause volume spikes in certain waveforms, but I'm not sure how effective that would be?
Something else that might be useful is a slewer? On the other hand, I can't say I've ever felt a real need for one.

Would like to know what your thoughts are on these ideas for a third row. Feel free to roast me over the decisions on the two rows I've already got!

Oh and just a patch I loved on the IDM/breakcore side of things that might be worth trying for any of you: I have the Warps on the obligatory Parasite firmware, patched a Korg ES-1 into the ADDAC200PI then through the Warps on Comparator, then back out. Results in extremely aggressive sounding drums!

Edit: And some more modules I have considered:
Befaco Rampage
NerdSeq - It would be nice to be able to play this portably if I don't use a FH-1/2 or Shuttle Control. my primary DAW software is a tracker so the interface is familiar to me and it has enough channels.

I'm open to ideas of different modules than the ones I've mentioned and prepared to accept I may have some ideas which don't make sense here. I haven't felt like there's a particular module I've got that was a mistake (though I don't find I use the Dreadbox WL Divider much) but if there's any I should really get rid of I could accept that too.

I can't answer all of your questions. But here's what I have.

Compare the NerdSeq to the 1010 Music Toolbox sequencer. See if one workflow is better than the other. I think the Toolbox will be better for instant gratification but the NerdSeq will be better for building songs.

The Expert Sleeper FH2 requires configuring before you access it with your DAW. If you wish to reconfigure it, then you have to quit out of your DAW, open the browser interface, make your changes, load up your DAW again and see if the changes are to your liking. You can make changes via the module's front panel. But the menu'ing system isn't that intuitive. That's been my experience. I would take this into consideration unless they've improved the USB interface.

Instead of a 2HP LFO, have you considered a 2HP switch instead? Being able to switch between different modulation sources might be more useful than another LFO.

Slew limiters are great. Being able to plug in a simple CV sequence and then slew between the values at a slow rate might be tasty for drones. The Rampage or Maths work well at this with individual controls over rising and falling CV. Both can also be used as a basic envelope generator as well as having comparative or even mixing functionality (Maths). If you have the space, one of these is a great utility module to have about.

One other device in the "slo-mo" modulation realm comes to mind: the 4ms Pingable Envelope Gen. This is capable of rise/fall cycles of a half-hour plus, and has clock-based control over cycle times with either an incoming clock or an internal clock with tap tempo. Adding one of Nonlinearcircuits' low-speed chaotic sources might also be of some use here.