Now that I've actually converted my 3.5 Eberron games to 5E and have players with dragonmarks, I've been looking at this more than before. I still like the Heroic Traits idea I had before, but I don't think I really figured out how I wanted to balance it until right about now. For right now, I'm testing this method: each character can have a single Epic Boon (DMG pg. 232). These boons must be worked out between player and DM. On top of that, the boons are modified from those presented in the DMG to be acceptable for lower-level characters. Follow these general criteria:
1. All boons require a use of Inspiration in some fashion.
2. If the boon has a short or long rest recharge, the recharge remains.
3. If the boon has an at-will spell, it can only be cast by using inspiration.
4. If the boon provides a permanent feature, using Inspiration grants you this feature for 1 minute.
This, of course, still isn't perfect, but I think it captures what I was really after: the ability to represent dragonmarks as a subset of a larger system of heroic abilities, but one that is separated from pre-existing game mechanics. With the above limitations, new boons can be crafted to fit any dragonmark. For example:
Mark of Scribing
You can cast comprehend panguages at will, without using a spell slot or any components.
My one player has a Siberys Mark of Storm and I'm essentially using the following:
Siberys Mark of Storm
As an action, you can cast Storm of Vengeance (DC XX) without using a spell slot or any components. Once you do so, you can't use this boon again until you finish a long rest.
Now, my players are level 12 currently, so while this is powerful, it's not too far off from how Siberys marks were in 3.5. That's simple, though fairly limited, and it only really captures the Least Mark. For higher marks, you can start tacking more on to the Boon or create new boons and let them have multiple boons (though this latter option might be risky). What I might consider is having stages of boons, so that your boon increases in power as you level. The normal Boon of Fortitude from the DMG increases your hit point maximum by 40. That's way too powerful for a low level character, but you could start small with an extra 10 hit points and then increase it by 10 every 5 levels or something. We could even use the cantrip level progression for this, even though these are clearly more powerful than cantrips. Epic Boons are just that: epic
, and I feel that's what dragonmarks should feel like for players. Even the low-level marks should make them feel special; like they can do something others can't. Sure, there are other people in the world with the marks, but shouldn't a player get something more out of it?