They have a conundrum to deal with in reference to feats. How do you deliver content, when they are optional? It makes more sense to treat dragonmarks like defiling in dark sun, or wizards/moons in krynn, where it is a background choice to gain entry. I also think dragonmarks should be a new type of rune magic, but I am not sure if that goes against canon. If they can be treated as rune magic, then any spells gained can not be used as additional spell slots for a caster. However, if a caster decides to learn rune magic (another flavor/background choice) then they can be viewed as additional slots. The downside is the casters body is covered in ruins, and some may view that in a negative fashion.
I'm not sure I understand your thinking here.
There's no real "conundrum" with Dragonmarks as Feats, because there's no guarantee that all "optional" rules are "optional" in every single setting. It would be madness to make that guarantee, and that's why they've not made it. Equally, even if you disallow all other Feats, you can easily fit Dragonmarks in.
As far as I know, Defiling and Moons haven't been described in 5E, so we have yet to see how they're treated, and further, I'm not sure if you're familiar with Eberron (kinda seems like not), but Dragonmarks aren't a "caster thing" or a "wizard thing", they're an "anyone thing".
But what I really don't like, is any way to offer new ability to martial classes, that is similar to spells. It appears these supplements will offer all classes content via backgrounds, feats or magic items, or exclusive content to spell casters via spells.
What are you saying here? It doesn't make a lot of sense grammatically. I mean, I can see two main possibilities:
1) That you're wholly opposed to "Martial*" characters ever getting any abilities even arguably similar to spells, by any means. That's fine, I guess, but you're in a sub-group of all players, and what is "spell-like" is hotly debated.
Or
2) Going in the opposite direction, and assuming you've missed a couple of words out, you're disappointed because they appear to only be going to deliver new content via Feats (which are supposedly optional), Backgrounds, Magic Items or Spells, and thus "Martial" classes don't have a good vector for adding content. In which case, yeah, that makes sense and is definitely a problem* in the longer-term, just like it was a problem in 2E and 3E, wherein there were hundreds or even thousands of spells added, which could be used to give more variety and options for casters, even in a "live" game ("I found a spellbook!"), but where stuff being added for Fighters/Rogues/etc. was pretty much limited to starting characters (kits etc.) or vectored in via Weapon Proficiencies/Feats/PrCs which are not in 5E (and which had problems of their own in those editions). Whereas 4E simply added Martial stuff via AEDU and avoided the problem (which isn't to say it was superior, just that it solved this particular issue).
In order to deliver stuff for "Martials" in a live campaign, you'd probably need to have something like "techniques", where a "Martial" class could know have X techniques "readied" at a time (i.e. they'd been training in them, had them memorized), and you could learn techniques from masters, or ancient books or the like (really surprised this isn't more of a thing in D&D, given it totally IS a thing in fantasy, history and in real life).
* = Martial is no longer an actual thing. This is 5E, not 4E.
* = For people who like more official crunch content in their games - obviously not for people who are not interested in it, but they are presumably not interested in the whole Unearthed Arcana deal.
[MENTION=13009]Paraxis[/MENTION] - Not a bad idea, but I suspect very few players wouldn't want a Dragonmark on that basis, given it's a guarantee of certain solid magical powers, whereas magic items are always DM discretion, don't always require attunement, and even those that do may well not be seen for many levels. Though that said in Eberron one would expect to see more magic items than other settings.