Tried to work on this, but much of the on-hand modules are simply too big to be practical. Plus, I'm definitely NOT an advocate of building drum machines in Eurorack...and here's why:
An Erica Techno System = $4829
Actual used Roland TR-909 = $7000-ish.
Now, the problem here is that, for one thing, actual TR-909 prices should not be that close to a modular system. Worse still:
An Erica Techno System = $4829
Behringer 909 clone (RD-9) = $349
Now, that's the current clone of the TR-909. $4480 cheaper. With that, you could buy four WMD Performance Mixers...or 12.836 RD-9s. Behringer notwithstanding, that's a massive drop in cost.
Yes, I get it...modular presumably gives you more flexibility in sound design. But so will a drum machine with independent outputs per drum and a small rack of outboard processors...and you'd STILL come out ahead.
My suggestion is this: stop. Take a close look at the Erica rig, and start figuring out if you really DO need all of that, especially if it takes up so much room in the cab that everything else has to be a compromise of some sort or another. If so, leave it in its own case, then build up a second and smaller cab for a complimentary system that contains both synth voices AND processing for the Erica rack.
And think SMALLER. Huge modules are nice in a huge system...but in something like a Mantis, they SUUUUUUUUUUCK. This is a compromise you should get used to. If you want the big module functions, keep the following in mind:
1) It's going to kill space in the cab for a lot of things, potentially including the utility modules these big ones need.
2) There ARE smaller modules that do the same (or sometimes more) as the huge things. For example, that HUGE Vermona meloDicer...there's a few other modules that do the same thing (Stochastic Instruments' Stochastic Inspiration Generator, Catalyst's Time's Arrow, etc) but which AREN'T 34 hp. Nor are they $480.
There's definitely ways to make something like this work, both financially AND practically. But right now...hmm, not so much. Consider a separate system instead. Otherwise, this will just turn into a frustrating exercise in trying to fit modules into a rapidly-diminishing space for a pile of money.