Yes, but what sort of martial? "Martial" is a very broad bucket term, it's not a character concept. How are you going to discuss the mechanics of what a char can do, vs what it should be able to do, if you don't have a concept to relate it to ?
Should a martial be able to fly ? Should a martial be able to create earthquakes ? Should a martial be able to power word kill ?
How would I know how to make an assessment of those things ?
Should a barbarian of the storm be able to throw around lightning bolts ? Ooh, now that I can see, b/c it totally fits the concept. Let's have a mechanics discussion about how much dmg those lightning bolts should do and also a story discussion about how it happens - do primal spirits fight for/with them, do they channel the power of their ancestors, does their rage manifest in physical elemental form, etc. Should a barbarian of the storm be able to change into a dragon ? Umm ... no I don't think so, because I can't see any link to the char concept.
Should a draconic instinct barbarian be able to transform into a dragon ? Oooh, yeah that might work, let's a conversation about the mechanics of the dragon form to ensure it's balanced and a story conversation about how it happens - does their rage make them forget their identity and an ancestral one takes over, etc. Should a draconic instinct barbarian be able to throw around lightning bolts ? Umm ... well .... probably not because it doesn't seem to fit the concept.
See what I mean ? To me, in order to have any sort of meaningful dialog about mechanics, I would think we need to understand what sorts of char concepts those mechanics relate to. Not every martial character should be able to do every sort of supernatural, fantastical, magical thing that exists just b/c they're "a martial". So what should they be able to do ? Well, it depends on the char concept.
This is nigh on exactly opposite to how I see things. I'd much rather provide a set of cool things a character can do, and let the players and DM figure out the concept that makes it work.
Personally, i think the biggest "problem areas" for 5e martials specifically relate to melee, so powers
I'd focus on would be abilities that improve melee.
- I might include greater and/or more versatile mobility. Maybe actual flight, maybe teleportation, maybe more effective leaping or climbing or maybe just a faster ground speed.
- I'd also probably look at including tools to add options to spread or focus damage. The fantasy is to "cleave through enemies like wheat" or whatever, but there aren't many tools to support that fantasy within 5e. Like, if you ever want to kill more than 9 guys on a turn, its just not gonna happen for you. On the other end of the spectrum, there's the "crazy power in a single, focussed strike" fantasy which is similarly unsupported.
- I might provide tools more relevant to exploration. Maybe busting through castle walls like the kool-aid man, maybe causing gusts of wind with a flick of your weapon, maybe some version of enhanced perception, hearing heartbeats, feeling vibrations on the ground, smelling fear, seeing the future.
- It'd be nice if there were better, more available options for control/debilitation. There are 11 negative statuses (excluding incapacitated and unconscious). Of those, most martials have access to 2 (grappled and prone), and they might get access to one or two more through their subclasses. Does it really make sense that no fighter or barbarian can inflict "blinded", "deafened", "paralyzed", or "restrained"? That no rogue can inflict "poisoned"?
Note these were off the top of my head and are all concept neutral. It's just "stuff they can do". You certainly can organize them and give some version of warrior some of those things, and other versions of warrior different things. The point is to start with what they can do in a general sense, balance, iterate, and flavor to taste.
Of course. But this has nothing to do with whether the character is a martial or a caster. This is about getting DM buy-in to play a char concept that has implications for the shared world. You would need this regardless of whether you are wanting to play a martial or a caster.
Correct. It was your assertion that Iron Man could be effortlessly modeled by a D&D martial, an assertion you supported with a set of conditions that had near nothing to do with existing martials' class mechanics. And, as you said DM buy-in, has nothing to do with differences between casters and martials.
Sure, but this is just b/c that example happened to have a magic sword. The exact same thing is true of a Miyamoto Musashi WITH the Sword of Kas and one without. The one without can't do the stuff the one with it can do.
Fair. I think
@Cap'n Kobold cleared this up. IIRC the original argument was Arthur+Excalibur=Morgana, and this is ok since martials get more/better magic items than casters do.
But since our wannabe Arthur is missing the kit, he doesn't get to operate at Morgana's level of power/capability. And he had no control over this beyond failing to choose a spellcaster.
Ok, now
this is something we can have a mechanics discussion about !

What would be the big caster items ?
Staff of Power & Robe of the Archmagi ? +4 AC, spell attack and save DC - the spell save DC is likely to go from 18-ish up to about 22-ish
What about for martials ?
Pick any 2 of the following: Armour+3, Shield+3, Weapon+3 & Belt of Giant Str (I guess depending on whether you're going for offense of defense or a mix of both ?) - those are pretty impactful too.
Do you think they make way more difference than the caster ones ? I'm not convinced that's so.
The interesting bit about this argument is that it suggests that there might be an issue with the "martials get more/better stuff" argument.
That said, I think you've chosen to look at the items with the most easily quantifiable impact at the expense of the most transformative ones (with the possible exception of the belt). Things like boots of flying, goggles of night, several of the rings, some of the cloaks, luck blade, dwarven thrower.
The unifying feature for most of these that changes the way a martial is able to play is by giving access to some limited spellcasting.