it's not really possible to make competitive tf2 more interesting by using other classes, because heavy and engineer and pyro are literally monkey retard classes that are cheap and unrewarding both to play and to play against and should be removed from the game, and spy is only fun to watch when the other team has a collective IQ of 80 because it's an underpowered class designed around a game mode where no one ever communicates anything or pays attention. as for unlocks, most of them are just +stat -stat versions of either stock weapons or other unlocks. sometimes something genuinely different is introduced—the pyro gets a flare gun, the soldier gets shoes and banners, the scout gets an energy drink and a cleaver, the sniper gets a jar of piss and a shield, etc—but even then a lot of the concepts are kind of ridiculous. "throw this jar in the vague direction of enemy forces to get extra damage for no reason!" again, if a monkey could use the unlock, it's probably bad.
if valve wanted to make the competitive game more fun to watch and less stale here are some suggestions
remove the stupid classes. there should not be things in a game where you can do nothing but stand next to a payload cart and hold M1 and still win. that's like playing baseball with an aimbot and the rocket jumper.
remove the insta-kills. no spy, no sniper, no random crits, no loch 'n' load type weapons.
add some classes that have interesting skill-indexed abilities. like, what if there was a class with the ability to fly around the map in, like, a hang glider or some shit. that would be pretty interesting. add more projectile weapons with different abilities, since projectiles require more prediction than hitscan.
make it viable to not have a medic. make uber less important. (with engineer removed from the game uber doesn't need to be a hard counter to anything.)
make some competitive maps in other game modes. like reinvent CTF with a neutral flag both teams have to try to capture, or reinvent payload with a cart that automatically goes forward when the attacking team has a numerical advantage. TDM insta-respawn should totally be an official mode btw.
make some unlocks that actually change the way a primary weapon is used. a scout primary that's not a scattergun (maybe the nail gun from the concept?), a soldier primary that's not a rocket launcher.
(this is the considered opinion of someone who only ever played tf2 in pubs and was spectacularly bad at it)