Moonveil Regent looks like an absurdly strong card to me. This might seem like a weird comparison, but it actually does look to me like it would play a bit like Experimental Frenzy. In a mono-red aggressive deck, for instance, if you’re playing Moonveil Regent at the top of our curve, you will usually able to play it as the last card in your hand, or if you have more than four lands, play it with a spell to follow up. So, thinking about it in the context of a monocolored deck, the card already seems strong to me.
You could also play it in a deck with a lot of multicolored spells. There are still a lot of cards to be previewed and it’s unclear which colors it would work best in, but another thought I had was using Moonveil Regent to go with Bard Class. This is already making gold cards cheaper; if you’re paying zero or one mana to cast multicolored spells over and over, particularly if you’ve also been able to get Bard Class to the third level, you’d may just be able to rip through your entire library.
Lastly, we’re entering into a set where flashback is one of the major mechanics. One way you can get help from drawing more cards is, of course, when you draw your first land for the turn. Being able to play cards from your graveyard will allow you to play a land for the turn and keep going, presumably until you draw another land.
All this upside is a lot for the card, and not to mention that Moonveil Regent is also a four-mana 4/4 flyer that even provides a slight bit of value by dealing damage to the opponent upon death. This looks like a fun card to brew with and has the potential to be one of the more powerful cards I’ve seen so far in the previews.