Why are people running so few lands on Niv Mizzet? I get constantly landlocked on T4, that´s why I gotta run 36 lands + 11 rocks. You never get any mana issue?

I wouldn't say I never get landlocked, but its fairly unusual. With enough cantrips like Preordain, Ponder, Impulse etc, it lowers the chance of getting mana screwed. Its not infrequent that I'll Preordain, see two lands on top, and keep both of them.

Hey Deakmana! A card you may want to test is Whirlwind Denial. The 3CMC is rough, but the interaction with Baral may just be worth it. 1U clear the stack and loot a bunch of times seems pretty good.

Howdy Magorian! While Whirlwind Denial is ridiculously cool with Baral, outside of that it just isn't that great. When the stack is full of stuff its usually a bunch of counterspells pointed at one another, and this would effectively be doing the job of a regular counterspell at (but at 3 mana). There are times where that isn't the case, but I think that its just too inconsistent to include.

Nice deck, i really want to try it on my playgroup. Do you have any suggestions what card to change for heavy creature meta(especially gravedigger because no one in my playgroup play graveyard recursions deck)? And what do you think about card like evacuation, aetherize etc since this deck doesnt have a lot of creature/defensive card?

What do you think about mystic sanctuary? With 8 fetches it seems like the equivalent of 19 islands is enough to have it on most of the time, and it can be fetched to get interaction back on top at instant speed later in the game

Some folks I know are on it and swear by it. It can do tricks like fetching it out in response to a Niv trigger to get back a counterspell like you mentioned. I'm a little hesitant on it as I actively avoid getting Islands so as not to feed Carpet of Flowers too hard. Because I haven't tested it out myself I don't have a very firm stance on it.