I want to see more delay + reverb pedals. The best on the market is still the SA Collider, and everything else is a less capable or worse sounding thing. Unfortunately the delays on the Collider are not as good as its reverbs, so it can't be my one and only delay pedal.
For delay, there's one delay type that is always above others to me: tape delay. I don't really care for distorted tape sounds, but I love wow&flutter modulation, aging the tape etc and the overall sound of tape delay. For multi-head tape delay, there's no better than the Strymon Volante because it's so easy to mess with the tape head playback/feedback combinations.
For reverb, the type I use the most is plate reverb. Throw some modulation with that and it's lovely. After that, hall and room reverbs are something I use and spring is "meh". But I get its popularity so a multi-type without spring reverb might seem like an odd duck unless it's a specialty reverb ala Strymon Nightsky.
I use the SA Collider with my BluGuitar rig for reverb and it's the right amount of complexity for me. Ventris is too much - too many options, too many things buried under the hood. I use the plate reverb on clean/edge of breakup tones the most, and switch to room for higher gain.
I use the Strymon Flint V2 with my Mesa Mark V "big rig" and love the Flint for its straightforward simplicity. It always blends in with the sound instead of overpowering it. I think it could benefit from being able to become brighter, but otherwise I have few complaints about it. Perfect meat-and-potatoes pedal.
I don't care for super complex delays/reverbs like the Meris X series. The UI is not great and you quickly end up in the weeds with it. I liked the Strymon Nightsky because the majority of its functions could be accessed with a dedicated knob or button so it's fun to experiment.
For hardware features, I think separate stereo 1/4" in/out jacks are ideal, but for compact I'm ok with the Strymon TRS input + mono/stereo toggle switch approach.
For MIDI I'd go with 1/8" TRS jacks today unless the pedal can fit 5-pin DIN jacks.
I like Source Audio's approach to presets where you can easily cycle them and have a few LEDs for indicators up to 8 presets. It's a good compromise when there's no screen.
I like Eventide's approach to 2nd layer functions: Dedicated button to toggle between main and alt layers, with dimmer labels under controls for the secondary functions.
If I were to build a delay + reverb pedal, it would most likely be a bit larger (maybe a bit smaller than Volante) to be able to fit maybe 4 knobs per side. That way you don't run into the Collider's stupidity where for either effect, the knobs never point at the actual settings because you need to flick between the sides.
Alternative if it has a screen, I'd say 4-5 encoders with a screen showing the values. I think the IK X-series design is actually pretty good, the big mistake of the design is using 0-10 style encoders instead of infinite rotaries and showing the actual values on the screen at all times.