The biggest challenge I had was due to some MSX computers not exposing the YS sync signal that is required to control the superimpose feature on the V9990. There are ways to derive the required signal for the computers that don't explicitly offer it on the connector, but as some computers do that and some not, the complexity to research and find out if your computer offers the signal, change/make cables, etc is not just for the normal MSX user.
Wait, wait! The MSX internal Ys signal is not necessary at all to have a working superimposer on the V9990 cartridge.
The idea is to superimpose the V9990 over the MSX video and not the opposite. IOW, the V9958 will appear as an extra (and very colorful) background layer for the V9990 games.
The Ys signal used for this type of superimposition is generated by the V9990 chip itself (pin-91).
Anybody with experience in video to shed some light?
I took a look at the schematics of the TRH9000 and I'm sorry to tell you that that the analog part is dangerously wrong.
The CXA2075 or an equivalent chip cannot be eliminated. It's not only the TV-Encoder, but it also does the RGB buffer. The MSX VDPs cannot drive their signals directly to a cable, at risk of frying the chip's outputs.
If you don't want a TV-Encoder chip, at least a THS7316 chip should be present to drive the RGB lines. But I strongly encourage to have a CXA2075, since sometimes the user simply won't have a monitor with RGB inputs to use. So CVBS and S-Video outputs are always welcome.
IMHO, there's also no need to have the separate sync on the VGA connector. This will be a hindrance since most people will just use it to connect to a SCART input that requires CSYNC. Just connect CSYNC output to the pin-13 of this connector since this is accepted by the majority of the VGA monitors/projectors that accept 15kHz RGB.
Also, the LMH1980 clearly states that "The logic outputs do not have high output drive capability.". This means that they also cannot be connected directly to the VGA connector and would require a buffer chip for that. So, just get rid of the U6A, and use a NC7S08M5X (or 74AHCT1G08DBV) to buffer the CSYNC signal for the VGA connector.
Lastly, I'm not sure if your superimposer will be able to impose the V9990 over the V9958. The way you have designed is described in the V9990 datasheet as being able to sync a dual V9990 setup and, if the timings are different because a different video source is used, the V9990 might malfunction. Since the V9958 might have different timings, I would strongly recommend to do extensive testing before approving this design as definitive.
Last, the pin9 of the VGA connector should provide +5V, otherwise it will be a nuisance to connect this cartridge to SCART inputs. You need to supply that +5V via a polyfuse, to protect the MSX power supply against short circuits.
THANK YOU! 
This is some really good feedback, and it has motivated me to do another round of design on this board - taking your feedback into account. I still won't be going with a Composite Video connector though, but instead I'm using a buffered MUX, the AD8188, and an RGB-amplifier - THS7374 - to fix the issues you described.
I'm also cutting the entire Analog-portion off of the main board! It's going to be a two-board card, like a sandwich.
My new variation is called:
TRH9000D-AB
https://github.com/Doomn00b/trh9000.git
Please, if you could check it out when you have the time, then that would be great.
Cheers-
-n00b
Ah... yes, I noticed this early on while researching the fabled Passthrough as well - so I just simply opted to only go with adopting the official RGB din-8 connector, from the MSX2+ standard, and ignore the rest. You make me think about how to circumvent the issue though! ^^ By creating daughtercards for the daughtercard, haha! 
With the right design, we can decouple the Din-8 connector from the graphics-card, onto a socketable mini-board that contains the plug and some support-components, which will then have to be altered for a couple of different variations of the RGB-input.
That's a ton more work of course... but it would offload the worst thinking for the end-users - they would just have to keep check of which MSX-model they have, and get the corresponding daughter-daughter-card for their machine.
An added bonus, is that it would improve the durability of the Din-8, since that part can be unsocketed while inserting the cable - the smaller pcb in the user's hand will make it easier to control the insertion-force.
Hi guys, some remarks on the RGB input. I think the superimpose model should just feature a VGA connector as RGB input. Many MSX lovers will have more than just 1 MSX computer. Having to have different exchangeable mini PCB’s will be cumbersome. The problem you are trying to solve is easy to solve in the cable domain. Whatever output your MSX has a cable that ends in VGA can be produced. Cables are cheap and VGA requires low insertion force and can be tightened after.
And I have said this before even if you put DIN8 connector on it, the user will need a new cable (he won’t have a DIN8 to DIN8 cable). The same goes for any other connector. Only input that would make sense for not needing a new cable would be a SCART input. That obviously has many other issues (so not a good idea).
If you use two vga connectors your probably can have it slim, and vga to scart connectors are easily found as well... As someone that doesn't use scart and have a 15Khz vga monitor, I would rather have it slim... But then, I already have a 9990 from Tecnobytes and probably wouldn't get or amek a new one for superimpose, so it is just my thought but I'm no potential buyer or builder or user for it 
This project must not die! It's too cool to wither away... @Crisag - please, can you give us an update on your progress? It could totally help me as well, to motivate me to do an update for my own variation.
I actually have a half-done new version - the TRH9000D-AC - it foregoes the dual-board design and goes back to a single board - the tolerances are truly insanely tight though - the analog part is done, but the Digital part... is going to be a pain. Soo many lines to draw.
se two vga connectors your probably can have it slim, and vga to scart connectors are easily found as well... As someone that doesn't use scart and have a 15Khz vga monitor, I would rather have it slim... But then, I already have a 9990 from Tecnobytes and probably wouldn't get or amek a new one for superimpose, so it is just my thought but I'm no potential buyer or builder or user for it.
Lol! Well, even so, thank you for showing interest in the project and giving advice.
Hi guys, some remarks on the RGB input. I think the superimpose model should just feature a VGA connector as RGB input. Many MSX lovers will have more than just 1 MSX computer. Having to have different exchangeable mini PCB’s will be cumbersome. The problem you are trying to solve is easy to solve in the cable domain. Whatever output your MSX has a cable that ends in VGA can be produced. Cables are cheap and VGA requires low insertion force and can be tightened after.
And I have said this before even if you put DIN8 connector on it, the user will need a new cable (he won’t have a DIN8 to DIN8 cable). The same goes for any other connector. Only input that would make sense for not needing a new cable would be a SCART input. That obviously has many other issues (so not a good idea).
Eyy, cheers for the reply mate! You know what? I think I will do what you said - I'm still, deep down, a bit against it, but not for any logical reason - it's mostly because my mind is stuck in the msx-standard that uses din-8 - and because I chose a Din-8 for my... ULTRA-MSX mini-itx computer!! ^^ Eh heh heh... but it makes way more sense to just go with the vga-connector on that too, instead though.
Still.. I must cry a bit, over spilt milk, because I made open source 3D models end everything (of Din-8), to prove my dedication... 
