Because they might not be canon but the knightworld list needs a bit of variety and people love to scratch-build their own models anyway.

All same stats as Paladin/Errant except for the weapon configs of course:

***Belier Knight (belier means battering ram in French apparently)
2 x Flamestorm Cannon - 15cm, AP3+, Ignore Cover (should it give ignore cover to FF too? The land raider redeemer flamestorm cannon doesn't somehow)
Siege Hammer - (assault weapon), MW, +1EA (hammer, or drill or whatever works)
This knight variant is to help out with the glaring problem of enemy infantry using terrain, especially buildings, to hide from the knights. It doesn't completely fix the problem, but helps out so at least there's a chance for the knights.

***Pavise Knight
4+ Armour save instead of 5+.
Thick Rear Armour
Twin Assault Cannon (30cm, AP4+/AT4+) instead of Knight Cannon
Chain Fist as normal
This knight is meant to provide cover, at least taking the first hit of a volley, for the other knights. Could make the errant more viable as it makes it easier to advance through enemy fire to get close. But is the small weapon downgrade (-15cm) enough to warrant the higher save?

***Culverin Knight
Battle Cannon, 75cm, AP4+/AT4+, Sniper
(no assault weapon except for the shock lance)
A 'sniper' knight. Increased range and sniper probably makes up for the fact of how vulnerable they become to being charged in close combat.

***(no idea for a name) Knight
Mini Burner (15cm) small arms EA+1 OR (close combat) assault weapons EA+2
Chain Fist as normal
A firefight-ish knight. Due to the way attacks work for WE, this chap if charging could get 5 attacks in close combat:
- 1 normal from being a DC1 WE
- 1 normal from shock lance when charging
- 2 normal from mini burner
- 1 MW from chain fist
Still, he's giving up a lot by not having the knight cannon or any ranged weapons at all.

Any thoughts?

_________________

I'm curious as to how well the shield knights would work given the way hit allocation works, it would only be good against 1-2 attacks as any significant shooting attack would overlap onto the rear knights anyhow wouldn't it? - or are you thinking of a group of them bunching together to form a LOS barrier as well?

Whilst the "Sniper Knight" sounds good, I'm not sure it fits with the theme of the knights, it feels more Tau or Eldar.

I like the idea of customization within the list however my personal opinion would not be to create extra "fixed" unit types but allow the custodes to take scout titan weapons and perhaps some minor upgrade/options for the characters.

For example:
A Castellan with an inferno gun is similar concept to the belier and good for clearing out infantry.
Upgrades could allow you to take a mix of weaponry (baron cannon, single shot laser destroyer, vanquisher or even a knight sized shotgun/flamer/flak cannon maybe?) and/or add abilities such as inspiring (a banner bearer) or short range AA (single multilaser on the carapace?). Obviously though these upgrades wouldn't be on mass but scattered across the handful of characters in your army.

Anyhow thats just my 2c