I decided to outline a few of the ideas I had, partly so I don't forget them, and also to see if anyone has some other suggestions :)
First of all, I've found a glaring problem with the head so graciously provided by the Dom/Rick Dom:

No Zeon suit should ever dare to make an appearance without a swanky chinstrap. I'll have to find some tiny tubing somewhere to make this happen.
Moving down the body, the chest area doesn't need a lot of work, but there's some stuff to be done on the back. First of all, I'll actually have to attach the backpack completely (I'll probably cement it on).

Since I've decided to equip this bad boy with two Heat Sabers, I'll also need to provide it with an extra attachment. All I'll have to do is drill a hole and maybe add an extra polycap on the inside to get a better fit. I also want to copy the hook itself with a putty mold so that they're identical on both sides.
And of course, the thrusters. I'm probably going to see if I can fit a size bigger, since these look a little on the small side to me (They're off the Dom Tropen, which is mostly a land based suit).
Since I already want to replace the thrusters. . .

Bigger thrusters = more output = more fuel = bigger fuel tanks. Plus, fueltanks look badass. I'm not going much bigger with these (not to like Sinanju scale), but I'd like to go at least 1 size bigger.
Moving sideways, we have the Dreissen arms. I love these badboys for the small 3-barrel beam cannons mounted in the forearms.

As you can see, they sort of lift open (it's not as glamourous on the model itself, you lift the armor plate off and place it on a higher peg) to reveal the cannons a little more. Except this doesn't look like it's actually necessary. They only shoot forward and they seem to have plenty of room to fire from, so I've decided I'll probably cement the armour plates together for a nicer overall look.
Another thing I like about the Dreissen arms, is the ornate Zeon markings present on the 'sleeve'.

I like it so much that I've opted to destroy it entirely, and fill in the area within the lower sleeve outline. The raised surface I'll be left with will hopefully look quite good, and make it look a little different from the Dreissen (since I want it to be sort of a precursor to it). The surface should also be big enough for a nice Zeon decal or something.
The arms also looked a little long originally, so I've taking the upper arm plates of the Rick Dom II and replacing the Dreissens upper arm with it.
It needs a little more work to fit properly, but the arms at least look like they belong to this suit now.
For the legs I've taken the thigh sections of the Rick Dom II, and the lower leg section of the Dom.

As you can see, these don't really co-operate. I'll probably see about cutting out the bar in the knee of the lower leg and fitting a polycap in there.
After looking at the legs a little longer, they do seem to look a little plain and barren compared to the rest of the suit. Someone suggested to me to add some external thrusters to the legs, which might be a good idea. I've also come up with this, in addition to the thrusters.

additional thrusters require additional fuel tanks, of course
The problem here is that unless I can find a way to steal some leg mounted thrusters somewhere, I might have to have a go at scratchbuilding them, which is something I've never attempted before.
I do have one more question to ask, and that's about replicating this little part (the handle of the heat saber).

Since I'm giving him two, I'd like both of them to be identical. This particular Heat Saber came with the original Dom, and is the nicest of the bunch. Since it's got sort of a 'claw' thing going on, I'm assuming that I can't easily recast one with putty. How else could I do it?
Hopefully this makes at least a little sense, I'm definitely all hyped up and am gonna cut into this baby very soon!