The design of the Fighter class limits what the fighter can be. It can be DPR. It can add a few tricks, it can even
actually cast spells, but those tricks are really only viable so far as they support that main function. The 5e fighter is a beatstick, a very good beatstick, and one available in several styles, but still a beatstick. Trying to shove it into any other function renders it sub-optimal or even non-viable.
Which, is not, by itself, a bad thing: It's strongly reminiscent of the 2e fighter, which was a damage king, and exactly what a lot of folks want out of the class.
With the fighter so much more limited than it was in 3.5, and there being so few non-supernatural sub-class options (only 4-5, among Fighter, Rogue, and maybe, Barbarian - out of 38 sub-classes among 13 classes in total), new 'martial' classes are a must.
The Warlord is the obvious one to start with, since it was in a prior-ed PH1, and can fill the critical 'healer' role (even though it never literally healed, just restored hps via inspiration), enabling functional parties even in low/no- magic settings.
Then there's the Scout from 3.5, a more militant/wilderness rogue.
Then there's all the interesting/fun/crazy builds you could do with a Fighter in 3.x/PF...
...and the one you couldn't: a Defender, like the 4e Fighter, or 3.5/Essentials Knight or the 3.x Devoted Defender/Dwarven Defender/etc PrCs...
..then there's Bo9S.
A Duelist/Swashbuckler/non-mystical-Martial-Arts-Master might be a good addition, too.
Once we have 4-6 mostly-martial classes in total and 20 or 30 such archetypes, we might consider further over-stuffing the game with yet more casters...