If you were to design a tone arm, what would it look like and feature?

No matter the design, it would have a removable head shell. Many companies now have that type as standard on their most expensive models.  There is not terchnical reason for not designing the arm with one, but old traditions die hard.

Every connection in a wire creates eddy currents and micro-arcing at the connection interface. This distortion is particularly harmful in a tone arm, owing to the fact the signal from the cartridge is the most highly amplified in all of audio. 
 

The same thing happens with physical vibration. Waves traveling up from the cartridge, instead of being uniformly and smoothly dissipated along the arm wand and into the body and base, are reflected back at the head shell/arm tube interface. Which is aggravated by the need for a fastening mechanism.

So it turns out there are a great many technical reasons why detachable headshells (and arm wands) are a bad idea. 
Unless of course you value being able to readily swap cartridges enough to make it worth the sonic sacrifice. In that case you really should just use a two or 3 arm table. But there’s always people who want to believe in the free lunch, a much more likely explanation as to why we see this obviously inferior design even on high end arms.

The arm's rigidity, mass, bearing & damping all play a role in extracting your cartridges peak performance. The tonearm has to support the cartridge in its correct position over the entire record, while allowing it to move inward to the centre of the record and navigate any vinyl surface imperfections.

I really like the options a Removable Headshell allows for.

One option that the method allows for, is to be able to consider a variety of materials and weights for the Headshell to be selected for use as the interface.

Another option, is that with a little homework behind one, a selection of Cart's can be pre-mounted into Headshells and the Arm VTA/VTF can be recorded for each configuration and recreated at a very short turnaround if a Cart' exchange is desired. This is also advantageous if it desirable to keep Cart' > Headshells in a Cartridge Keep when not in use, as I do.

It is also easy to acquire a Headshells in a variety of materials and weights with a design that has a clamping collar with a fastening screw, that enables the Headshell to be loosened and used to align Azimuth to an optimum.

A few nice options pointed out that are available when going down this road.

There is also another side to Removable Headshell that is not so attractive, which is the 'Typical Design' that is seen on many of the Headshells available to be used in a multitude of Tonearms.

For the present-day performance levels of Cartridge being produced and the monetary values associated with them, it does not make any sense to mount them onto a Headshell that connects to an Arm Wand, where two generic devices to produce the connection are meeting and completing the overall assembly of the Tonearm.

Companies that have a long-term association with Particular Designs of Headshell connection have chosen to supersede the earlier designs with New Designs that are attempts to vastly improve the rigidity at the connection and substantially reduce flexion occurring over their earlier offerings.

Unfortunately, it would appear to keep the Company responsible for the New Designs, Marketing at a particular place in relation to competitors' offerings for the Market Place, these newer designs are not generic, and are limited to particular models of a Tonearm Wand only, this is a shame as it does leave the Older Designs needing a Bespoke Remediation if they are to be improved.  

As a person who over the past years has been very involved with being an individual, who has been trusted with being introduced to the designs of a few new produced tonearms. One of which is a substantially modified Tonearm, which does have a Removable Headshell and One that is a design I have been involved with from conception through to becoming a working model, that is a design that also has a Removable Headshell.

As a result of this involvement, I have had firsthand encounters at experiencing designs for removable Headshells and the improvements put in place for the designs for a Removable Headshell.

As a over exaggerated description a 'Typical Design' for Removable Headshell that  is commonly seen, is one that is behaving like a Flag at the end of a Flagpole. The Headshell, as a result of a lack of rigidity being engineered into the connection between Headshell and Arm Wand is functioning like a Flag when exposed to ambient conditions. For the Removable Headshell, this needs to be scaled back in its movements. It is Micro Flapping around as a result of the ambient forces encountered.

This movement, that is occurring, as a result of a loose fitting of the Headshell to Arm Wand connection is one that is with Latitude, there is an unwanted freedom, a freedom that has no pre-ordained calculation produced for it. The movement that is available is only present, due to the tolerances between generic parts. The non-standardised condition to be found at the connection, will allow for a Headshell to produce a movement in any direction the ambient forces dictate.

What is the worse is that these movements that are manifesting on the Headshell are at the end of the Headshell closest to the Wand and are at their best within the Male/Female connection at the coupling.

These movements when transferred to the end of the Headshell furthest from the Wand are an amplified movement and one that is taking place over the point the Styli is interfacing within the LPs Groove.

A Cartridge as a design has evolved over many years, and much of that design is to enable the Cart's function to be optimised, when known conditions are to be encountered. The conditions that are known are pretty much defined by the Cart' >Arm Compliance, Cart' Loading, as well as the impact of the Stylus within the Groove.

The Cart' as a design, using the knowledge I have gathered through investigation and discussions, is suggesting the Cart's in general have never been designed to be mounted onto a Structure that is non rigid, especially where the rigidity that is present, is to the point, it is able to produce a Circular or Elliptical Rotation as a movement generated from the supporting structure.  

Taking this a stage further, if a Tonearm that has a removable Headshell and the Wand has incorporated a 'Typical Design' as the method for attaching a Headshell.

The Headshell chosen might be a Big Price Tag Item, from a highly sought-after Brand. 

The Brand has a Generic Male Connection Part inset into the Headshell and the Arm Wand has another Generic Female Connection Part inset into the end of the Arm Wand. The connection between the Generic Part and Headshell or Arm Wand is already a point of connection that can have an interface tolerance issue, then the coupling of the two generic parts will bring further areas of concern for the interface and tolerances that are present.

With the above as a very common method used and the conditions being presented are a reality not fantastical, an individual then makes a decision to mount a Cart' that costs multi-Thousands. It is difficult to see how placing such a valuable item, with a specific design intent, is best served in use in such an environment, from my end there is strong suggestion there is a detriment present to the optimised performance from the Cart'. 

I don't confront the idea of mounting any Cart' on any Tonearm, it is the user of the equipment's choice for how they wish to select and set the parts up.

What I do confront is the notion that a particular method of selecting and setting up the parts is an optimised arrangement. 

I have plenty of knowledge of Fixed Headshell Tonearms and do not use them anymore, as an impression made from the experiencing a Modified Tonearm with a Removable Headshell has been very persuasive in my decision made to supersede the fixed Headshell Tonearm and not keep them in use.

As for the Removable Headshell Tonearm I use, I have been instrumental in encouraging a design to be produced, for a much-improved rigidity at the Headshell to Wand connection. I have heard this New Design as a A/B comparison, of which the New Design for the coupling has been a night and day difference to the betterment and there is no reason to want to remain with the original design, even though it is a little quicker to make a Headshell exchange.   

I have also been instrumental in encouraging a Tonearm designer / builder to consider a design for a Removable Headshell, that is one that is not an overhanging attachment that can have flexion and issues with rigidity as a result of the method of connection.

The design for this Tonearm has a Detachable Headshell that meets the brief and is without doubt from my observation and understanding the value of the known machining tolerances, a Removable Headshell that has an extremely rigid interface, and possibly functions as a much improved method than what is on offer from many fixed Headshell designs.

The times I have been present for the demonstration of this Tonearm, has a Tonearm in use, that is undoubtedly a device that has a Mechanical Function that is extremely accurate, of which a proportion of this is attributed to the design for the Headshell connection.

I encourage an individual with a curiosity, to look into this further, as the outcome of making a change to improve the mechanical interface at Headshell to Wand coupling, can be a night and day difference and one that is for the better.