The key to making Fighters "non-boring" is to figure out why your guy or gal does what he does...and how. Give him a personality.
One of my most memorable PCs of all time was a 1ED Fighter named Bear. I made a deal with my DM- give me maxed out physical stats and all of his mental stats would be 6-7s. Bear was a gentle giant, he fought because he was trained to do so by those around him (what else was he going to do?). Despite his nature, though, he had hooked up with a manipulative thief who treated him well...in order to have the most loyal and dangerous bodyguard he could find. That thief was- to Bear- brother, father and God all rolled up into a diminutive package that was his only true friend.
The thief eventually took something he
really shouldn't have, and the City Watch boiled out of their barracks like fire ants from a kicked-over mound. As the party fled, the thief told Bear to protect him...
As they crossed a river on a narrow bridge, Bear turned and faced the entire Watch himself, taking down one after another until he died. By then, the party was safely away.
Other fun fighters had unusual intellect or wisdom in the other direction, oddball weapons, quirks & taboos, etc. It wasn't necessarily about the fight.
has anyone tried any multi-class fighter builds that ended up being interesting?
Sure- dozens of times, in every edition!
Examples from 3.X:
1)
Johnny Bones: a NE Ftr/Thief, done to conform to 1Ed type class stats. He was a bit of a street-thug who fell in with a good crowd. Didn't stop him from bullying the party mage, though. Points for style included TWF with Rapier & SS, and studded leather armor with skull-shaped studs.
2)
Adragon Von Basten: a CG Ftr/Sorc. The son of a high-class courtesan and a Half-Dragon mercenary, he worked as a bouncer in the brothel until he was forced to leave because he killed an incognito noble who raped and killed the woman he loved (a seamstress who worked at the brothel). He wore Scale armor & shield, fought with a Maul. Most spells he knew had no somatic components. Cast almost no spells in combat- instead, channeling the spell energy for a lightning BW via the Draconic Breath feat (CompArc).
3)
Sister Shrike: a LN Ftr/Monk/Kensai. Dex-based, used AoOs & FoB with a Greatspear (Pole Fighter feat lets it be used as a monk weapon), Monkey Grip, and potions of Enlarge to deal out some serious hurt to some dragons at a Dallas Gameday hosted by GenX Games. Battle was her prayer.