These ideas could be used across a whole new regiment, to create individual squads for variety, or even to make character models stand out. With the new 6th edition Allies rules, the Guard have become more popular than ever - cheap troops and reasonable HQ costs make them a solid choice.
This post will feature some of the ways in which I've converted my own or commission projects over the years, and will mainly focus on the Cadian sprues - despite their age, I still find the Shock Troops kit to be one of the most useful GW produces!
1. Using backpacks
This is an obvious one, but strangely can change the look of a squad instantly! The standard Cadian and Catachan infantry sprues don't come with packs, but the heavy weapon sprues do. Alternative bits are available from Maxmini, Secret Weapon, Forgeworld and other suppliers.
More exotic backpacks can make for a more exotic regiment: take a look at this example - rebreather packs from Westwind Miniatures for my Savlar Chem-Dogs...
2. Consider Warhammer Fantasy Battle kits
With the advent of more plastic kits, don't feel constrained by just 40k plastics! Many WFB plastics can be used as conversion fodder. Capes, swords, archaic pistols and more are available across the WFB range.
A simple cape conversion, making for a more noble-looking Cadian officer...
... through to noble-looking Royal Guards, all made with parts from the Empire Pistolier kit...
3. Use a headswap
... or a leg, torso or even arm swap! With so many manufacturers now making 28mm-compatible bits, and the variety of plastic GW kits, it's never been easier to change the look of your models.
Here's an example using the awesome Forgeworld Cadian respirator heads...
... then another using a head from Pig Iron productions...
4. Think outside of the box
Don't feel constrained by just using a particular kit for a particular purpose. If you want your loyalist Guardsman to have a pistol from the Space Wolves kit, then use it. If you want your Grey Knights Grand Master to carry a sword from the Chaos Space Marines sprue, then use it.
Here's an example I made - a Cadian Penal Legionnaire made with parts from the FW Renegade Militia set. It required only minimal conversion work (filling in Chaos symbols with green stuff etc).
5. Variant uniform patterns
By using your own paint scheme, you can individualise stock models immediately. This is especially the case if you do something completely different to the standard GW scheme.
Here's an example of a Harakoni Warhawk from an army I sold earlier this year - note how the fatigues and armour create strong contrast, and bring the model away from any suggestion of it being "just another Cadian"...
6. Consider abhumans
Do you Guardsmen even have to be fully human? Older editions of 40k have examined the use of mutants and abhumans in the service of the Emperor, and the new 6th edition ruleset brings along the suggestion of possible abhuman soldiers.
Your abhumans could fill any part of your army - perhaps a unique veteran squad, or a special weapons squad (after all, maybe those demo-charges will misfire, and surely better to lose a Beastman than a human soldier!). I went so far as to build and paint an entire platoon of Beastman soldiers.
7. Think about your bases
Basing is an area overlooked by many hobbyists, but it really shouldn't be. Bases can really tell a story. Using resin bases, you can quickly establish the fighting conditions of a whole regiment. If you'd rather concentrate an individiual models, then using a special base for a character model can add to the gravitas of your warlord.
Here, I've created a basic slate base for a Savlar commander model - note how by adding height to an othersise dimitutively-scaled model, already he looks more menancing...
8. If all else fails, go the Forgeworld option!
Alright, this is a bit of a cop-out, but if you want an army very few people will have, you can always consider using whole Forgeworld models. They make amazing models - with highlights including Death Korps of Krieg, Elysian Drop Troops, and specialist Cadians.
Here are some Elysians I painted a few years ago, in standard wilderness-pattern gear...
9. Bringing techniques together
Of course, you can mix and match all of the ideas raised above in creating something really unique.
Here is an example of my Mark I Chem-Dog rough riders - using mounts from the WFB Chaos Marauder kit, torsos from the FW Renegade Militia set, and bits from several other 40k sets.
Hopefully that will give some food for thought next time you build a Guard squad - please feel free to leave comments!