Room on the broom or The Worst Witch. If you have a witches costume from Halloween this is a super easy option. And if your child has the same multitude of stuffed toys mine have you could assemble the dog, cat, frog and bird in a bag/cauldron.
Victorian outfits can be used for Oliver Twist by Charles Dickens and Hetty Feather by Jacqueline Wilson
A football kit is a bit of a cheat but ties in nicely with Frankie's Magic Football by Frank Lampard which my son loves.
Crocodile onesie (ours was from Sainsbury's last year) suitable for Roald Dahl The Enormous Crocodile, The Selfish Crocodile by Faustin Charles and The crocodile who didn't like water by Gemma Merino.
Dalmatian onesie which of course would suit 101 Dalmatians by Dodie Smith.
Road Dahl's BFG is a great World Book Day costume and can be easy to make. The ears are just a headband, pink paint and card.
Click the link to see full details to make the costume.
How to make BFG ears, horn and waistcoat
Finally Harry Potter. Again if you have a cape from halloween hanging around that would do over a school shirt/trousers. Some cheap black frame joke glasses and the all important lightening bolt scar. This can be drawn with face paints or your own eyeliner/lipliner. A lot of shops have full Harry Potter costumes fairly cheap if you'd prefer to purchase the whole look.
Happy world book day!