upon
a red wheel
barrow
glazed with rain
water
beside the white
chickens.
— William Carlos Williams
Suggestions should depend on your giving us some direction.

What are your tastes? What's your guitar/amp/style-of-music?

For someone in a Pantera tribute band playing through a Krank amp

I'm going to give completely different suggestions than what I'd suggest to someone running through a Blues Junior playing along to Youtooblues vids in the basement

.
Your response to Owlexifry's suggestion finally gave us some indications of your intentions/preferences; and you let us know you like phaser, that you want to try something new other than that ol' 45 and that you want a tremolo...

Now we're getting somewhere.

I can now drop the bass-specific suggestion of Envelope-Filter > Compressor > Fuzz > Flanger.
So...
1) YATS
2) Distortion
3) Tremolo
4) Delay
Given the size of the enclosure, you could squeeze 5 effects in there if one (or two) is (are) a small PCB (1590A-sized) such as a boost/fuzz.
Boost can push a tube amp into more saturation, add fuzz for leads...
The enclosure's big enough at the back that I'd put in switching jacks for each effect, that way you could insert an external effect-chain before or after any circuit within the combo. With nothing plugged into the inserts, the signal just flows through whatever you've put in the box, of course.
Maybe that's overkill and you just want one or two inserts at strategic locations such as between the distortion and tremolo to plug in a Phase45.
1) YATS
Most screamers are 3-knobbers, most of Robert's PCBs are standardised for 1 drill template — so if it's a 3-knob yats, you could add in quick-connects to swap out different YATS, or any 3-knob OD;
most of the 4-knob ODs in Robert's offerings share the same drill-template too. So really, it's a question of do you want a 3-knob or a 4-knob YATS/OD?
Or if you're okay with a few extra holes, drill for the Frost Drive (six holes) and a standard 3-knob YATS could still be bunged in at a later date or vice versa. OR you could leave a "ghost" pot in place if you don't like holes.
It's not a quick-change, but having the quick-connects on the wires means not having to bust out the soldering iron when changing out the YATS, just a matter of taking off the knobs and loosening 3 (or 4) nuts.
2) DRIVE 2/DISTORTION
Marshall in a Box? A Rat? Why not go with a Six-knob solution, and again have the quick-connects on the wiring. Then you can swap out the board in this slot as well:
Pestle (EQD Zoar) / Blue Shoe Gaipan / Dingo (VFE Alpha Dog) / Thermionic (Friedman BE) / Paragon Mini (KoT) / Plaid Preamp ( Funny Little Boxes 1991) / Mofeta (EAE Model FeT / Boogie Monster / Bayonet Fuzz (BoE Musket) / Blues Captain (MI Blue Boy) / Captain Bit (oct-fuzz!) / Chela (EQD Talons) / Death Cap (Fuzzhugger Doom Bloom) / Sea Monk (VFE Merman [klon]) /
Ghost-pots would open this up to some of the 5-knob options for even more versatility, or add a mod to the 5-knobber to make it a 6-knobber.
You could even sub out a Delegate compressor in this spot!
OR SMALL PCB (to make room for adding a boost) one-knobbers:
The Crankshaft (EQD Speaker Cranker) / El Sol (EQD Acapulco Gold) / Aft (EQD Bows) / Mini-Muffin
3) TREMOLO
Why not Nick's Arachnid suggestion? 8 hand-picked FV-1 effects including Tremolo.
If not, then choose a flavour...
Pendulum for Harmonic Trem / Woodpecker for Hard Chop / Special Porpoise fuzz-trem (Catalinebread Antichthon)
4) DELAY
Arachnid AGAIN, but with all 8 FV-1 algorithms being some form of delay/reverb.
Or continuing the quick-disconnect theme, these share the same drill template:
Bugg FX Daydream / D3lay / Unison Double-tracker
BOOST
Compact one-knobber with a flip-flop switch to put it at the end of chain for a clean solo boost/driving the amp,
or at front of chain to push dirts 1 and 2 in the chain after it.
Amentum (EQD Arrows) / Black Tiger
Thanks for the inspiration, lrgaraujo, I might have to do something similar to what I've just suggested!