It is my interpretation of canon that not all spells are inflexible per se, meaning some can bend and move
Not all spells travel in just one direction. For example: Protego Totalis, Repellum Muggletum, Muffliato, and Priori Incantatem create what I can best describe as a dome-like effect, encasing persons or an area. The first two spells are protection spells that Hermione uses in Deathly Hallows, Muffliato is a spell that keeps those around you from hearing a conversation you're having with others (Harry, Ron, and Hermione used it in Half-Blood Prince and Deathly Hallows). We're probably all familiar with Priori Incantatem and how it created a cage of light over Voldemort and Harry in Goblet of Fire. Hominum Revelo is a spell that Hermione casts just inside the front door of No. 12 Grimmauld Place, which reveals there are no other persons within the entire house. In Prisoner of Azkaban, Harry produces a Patronus, which moves freely about once its in corporeal form. The Patronus doesn't require that a person aim at another or a being. Harry Accioing his broom, as stated in your question, is a great example.
A literal example of a directional spell is Point Me, which tells the caster where he or she is.
There are many examples of spells aimed at a specific person or being being deflected or reversed. In Chamber of Secrets Ron's defective wand shoots the slug-vomiting curse back at him, rather than straight forward to Malfoy. In Goblet of Fire, Harry's spell connects with Voldemort's and keeps it at bay, forcing Voldemort's spell into a kind of stasis. When Harry jerked his wand up, breaking Priori Incantatem, Voldemort's Avada Kedavra spell did not continue on toward Harry, so one could say Harry's Expelliarmus blocked Voldemort's spell. In Order of the Phoenix, Dumbledore's duel with Voldemort featured a lot of blocked spells and spells that did not hit their intended target.
But the headless golden statue of the wizard in the fountain had sprung alive, leaping from its plinth to land with a crash on the floor between Harry and Voldemort. The spell merely glanced off its chest as the statue flung out its arms to protect Harry.
Harry Potter and the Order of the Pheonix, chapter 36
It's also in Order of the Phoenix that Fawkes actually swallows one of Voldemort's Avada Kedavra curses, dies, and is reborn. Prior to Voldemort and Dumbledore's duel, the DA members battle the Death Eaters in the Department of Mysteries and curses are deflected, misdirected, and incomplete, and they certainly do a lot of damage to the Hall of Prophecy and the Time Room, breaking and destroying myriad items, and the spells stop once they hit a target (even if it's not the intended target). In Deathly Hallows, Hermione's Confringo spell cast at Nagini rebounds from inside Bathilda Bagshot's house and hits Harry's wand, breaking it.
Protego is a blocking spell that a person can cast against an incoming spell. I don't think it would work against Avada Kedavra, because in Goblet of Fire, Barty Crouch Jr. as Mad-Eye Moody states that there is no known countercurse to Avada Kedavra and that it cannot be blocked. It can, however, miss its target.
So, some spells do not travel only in one direction. Spells that do travel in one direction can miss their target and hit something else, which completes the spell. Spells can rebound sometimes. And they can be deflected in a number of ways.