No strict rules, only suggestions and ideas based on personal experience.
Mount drivers as close as possible. Use as wide BW drivers as you can find.
Paper membrane for clarity and speed.
Poly for smooth roll-off at the top end, but a bit coloured.
your choice of soft dome, metal dome, ribbon, all with low res freq., or compression drivers for low x-over, most importantly high SPL.
Use a tweeter with a waveguide.
Depending on the quality of drivers use a 1st order series x-over (my personal choice), with a BSC circuit in place.
Being a 2-way your mid woofers should not be any bigger than 6.5", unless you can cross your tweeter at around 1 kHz, maybe around 1.5 kHz (difficult, I know).
Many other things that I'm sure other people will suggest.
Thank you Charles, I should've guessed that on my own. Bandwidth is one of the reasons I build 4-ways. None of the drivers are stressed, so concert hall levels are still crystal clear.
Stanislav. Yes to series crossovers. I use a dual cascade x-over. I believe that makes it a second order? Here's a link I like. New Crossover Designs . Near the bottom is the 4-way Series-Shunt circuit. It is a first order. When I start my build (6-8 months) I'll start a new thread. I use a second order 4-way Series Shunt.
As the first two mentioned, for proper off-axis response, the tweeter crossover of a MTM system should be quite low. Your center to center spacing is about 20cm at a minimum, which equates to 1700Hz. The crossover should be below that. You might be better off making it an MT or TM.
The narrow vertical directivity of the AMT might come to the rescue here. There are some MTMs with AMTs and ribbons etc that work quite well because of the narrow directivity of the tweeter even if they don't follow the D'Appolito rules for vertical spacing relative to the wavelength at the crossover frequency.
Edit: Even with narrow vertical tweeter directivity 4.5 kHz is definitlely on the high side for 20cm CTC distance.
That changes the approach a bit.
One important detail to remember is:
the centre of your HF driver should be approx. 0.9 - 1.0 m above the floor.
That makes it a massive speaker, tall and heavy.
Maybe you have a forklift to move it around.
Study the work of the late John Dunlavy, first with Duntech, and then with DAL. He was a great advocate of 1st order networks and symmetrically arranged multi-way speakers. Duntech Sovereign Review
Also check Jamo and Dynaudio (Confidence), they made some similar speakers in the past.
Or maybe take a hybrid approach like Amphion Krypton.
Regarding x-over, series one will be very tricky with a 4-way, too many inter dependable relationships that will spoil your final result.
Series x-overs require very careful balancing of all the drivers in respect to SPL, impedance and phase variations.
When you include BSC in it becomes even more complicated.
Series X is best with 2-ways, and can be successful with 3-way if the other conditions are met.