Though he's not designed to stand on a standard 25mm base it's certainly possible to convert him to do so.
If you're building a Cannon Of Khorne anyway this not only saves you money because you don't need to buy a separate GW Finecast Herald Of Khorne, it also produces a sturdier and possibly better looking model.
Though it's possible to cut and re-position the legs of the Herald to fit directly onto a 25mm base, I preferred to do as little cutting as possible. Here's how I mounted my Herald Of Khorne on foot.
First I mounted the body of the Herald to the unused back of the throne that comes with the kit as shown in the photo below.
Next, I mounted the back of this part directly onto a 25mm base as shown in the next photo.
Finally, I finished assembling the Herald. I carefully clipped the chain he was holding from his left hand leaving just a plain clenched fist thrust forward. The finished model is shown below.
Notice how this cool looking scenic bit fits his wide stance. It also boosts him up quite a bit making him appear even larger next to regular Bloodletters and other models in the army. The angled mounting will also allow him to rank up better with other models for Warhammer Fantasy. Of course, I could have just as easily mounted this conversion to a 25mm round base instead.
With literally just a few minutes of hobby time and a little imagination I've made good use of the Herald I already owned by purchasing a Cannon Of Khorne. I may build another similar Herald since I have two Cannons Of Khorne, but with some variation in basing or arm poses so that they each look a bit unique.
Changing the pose can be accomplished by simple techniques such as cutting the left arm at the elbow and rotating it or changing the angle of attachment and maybe even adding something decorative and iconic to the hand like a skull on a chain. The angle of the sword can be changed by rotating the right arm through altering the angle of attachment at the shoulder. The positioning of the legs could also be altered by making some cuts and adjustment to their angle of attachment as well.
This minor conversion was simple and fun and yields a pretty dynamic looking model with a great scenic base. I suggested this to someone the other day when they were trying to figure out how to make their plastic Herald stand on a 25mm base and they instantly pounced on the idea they liked it so much, so my idea was "stolen" even before I got to use it myself!
That's half the fun of this hobby, sharing ideas with other hobbyists and seeing them get excited about them. And now I've shared this idea with all of you reading this. I hope it inspires a few great looking Heralds out there in the Realm Of Chaos.