Byrons_Ghost said:
My thoughts on Dragonborn would be to have them in the area of the Plain of Fire and the Great Waste, and to have been a servitor race originally created by the Nithians. Alternately, if we're sticking with the "remnants of ancient empire" idea, then they could be from someplace like Davinia or the Arm of God, and simply recently arrived in the Known World.
A couple of options for dragonborn come to mind.
First (and notably) would be to either use them as a substitute for the chameleon men of the Savage Coast (who are dragonkin and had a great empire in ancient times, before the aranea destroyed their civilization).
If you didn't want to do away with the chameleon men, you could still keep them there on the Savage Coast/Arm of the Immortals area, and have them be a related group that similarly fell into decline when the aranea cast their spell way back in the day.
Alternatively, the dragonborn could be the Carnifex, who are a race that is only mentioned (canonically) in module M5 that were a powerful saurian race that wielded magic and were such a potent threat that the Immortals arranged for their imprisonment in another dimension.
Same goes for the tieflings. They look like diaboli alright, but the mechanics might be hard to do (not sure what the tiefling's racial attributes are in 4e). Also, I seem to recall that diaboli were immune to Mystaran magic, which could definately make for some balance issues.
The diaboli immunity to Mystaran magic was removed in their 3rd Edition conversion, but I definitely take your point (it should also be noted that Prime creatures were similarly immune to diaboli magic).
I think the tiefling/diaboli comparisons are pretty good, and I hadn't thought of them myself. It would take some tweaking to fit, though, as Byrons_Ghost notes.
For anyone that's interested, there is an ongoing effort over on the Mystara forum at Wizards/Gleemax to compile what is basically an introductory guide (mechanics free) of Mystara, with an eye towards using it as a possible basis for a 4E "conversion" effort.
(I hesitate to use that word, since the 3E "conversion" of Mystara fell so flat, and I don't see this project as- hopefully- trying to scrunch and stretch mechanics to fit so much as just provide a comprehensive and singular descriptive source of the world that people can use regardless of rules mechanics.)