Big Mac wrote:a BECMI thing (with an implied Mystara connection).
More than implied. All the modules in the DA series begin (at least potentially, since some have multiple options) in present-day Mystara and involve the PCs traveling through time back to the age of Blackmoor. Mystara sources mention Blackmoor countless times, though more for its latter age of technology and its cataclysmic destruction than for the age of Uther when the DA series takes place.
The Inner Planes of the Great Wheel but bespoke Outer Planes,
d20 Blackmoor actually has a modified set of Inner Planes, adding the Elemental Plane of Wood and the Elemental Plane of Metal to the mix. Its outer planes are undefined, though the existence of the Abyssal and Infernal languages implies that demons and devils are in separate camps. There's also a Plane of Time somewhere.
The standard 9 alignments of AD&D/3e (although d20 Conan seems to dump this)
d20 Blackmoor has these.
Playable monster races (with LA penalties)
d20 Blackmoor monsters do have LA penalties, so if you wanted to play, say, an ash goblin you'd have that covered. There's information on balebourne orcs as characters. Skandaharians have an LA, even though they're human.
From what I recall of my Blackmoor book (which I've not got with me today) it does include deities (so might conflict with the DA series
The DA series has deities, too. In DA1, Zugzul is described as "the great god" and his high priestess is known as the Mistress of God. He was retconned as an Immortal of the Sphere of Energy in DA4.
The deities in ZGG's Dave Arneson's Blackmoor
sometimes have the same or almost the same names as Mystaran Immortals, so it's fairly easy to assume they're the same beings, if that's what you wish. There are other threads in this forum dedicated to the correspondences between the Blackmoor deities and the Immortals of later Mystara.
What are the main differences that 3e and 4e introduced? Are there a lot of changes to take into account?
We've talked about this a little in other threads. 4e added dragonborn and tieflings to the campaign and removed gnomes from the list of PC races since gnomes weren't available as PCs until later 4e. The way gem magic works changed because early 4e doesn't have schools of magic, so they had it correspond to energy types instead.