I do like the direction this is going in. The harmonies are nicely done; I guess there may be illegal parallels and such, but that's not too important. It begins very majestically.
I do have two pieces of advice, or constructive criticism to offer.
1. I think perhaps the ostinato melody itself in the bass is a little long. Or at least a bit too complex. I think a simpler bass line will allow you to vary that melody even more. Plus with a simple bass there will be more room for the upper voices to go crazy
. I suggest that you take out any notes or motifs that don't really need to be in the ostinato melody, perhaps by singing them to be sure that they all fit in perfectly... after all, the whole piece will be built on that melody so it had better be flawless! That said, I don't think it is bad; it really depends on how you develop your ideas later on... which brings me to my second piece of advice:
2. I think perhaps you could develop each motif you invent in the upper voices a little bit more. For example, you begin the first variation with a plain chord... don't think that the listener will get bored if you have a whole variation of mostly plain chords! Then you go to what I presume is 8th note motion. That could be extended too! I don't necessarily think you can't continue as it is, but you might run out of ideas if you have a new motif every bar.
So that's all just my opinion; it's a good start! Keep on writing!
And by the way, putting the ostinato in a higher voice is a very good idea for occasional variety, although you might want to save that for later on. As for the mirroring thing, it sounds intriguing, so be sure to develop that idea extensively too.