RisenDeep

+1 for Nightmare tribal :D

Streambed Aquitects seems as the best option to me because they are far too expensive for there abilities

You are playing UG and Metallic Mimic but not Deeproot Elite?

You could reduce Polymorph to benefit from a Proteus Staff that is also able to cast away opponents creatures without hitting any sacrifice triggers

Merfolk Mistbinder is a bad choice as 4 drop. You should instead increase Merrow Reejerey and add some Phantasmal Image. The reasons are that Merfolk Mistbinder truely pumps your creatures but dosent do anything more besides that. So you wont get Islandwalk from it nor is it untapping or tapping some cards. You will always have at least one potential lord in hand and so on the board so playing Image you will benefit from the lord anyways. Either it gives your existing lord Islandwalk or you have two untappers in play that can fast boost up your mana base by untapping two lands each time you just play a Merfolk card.

I also would remove some Sanktums and/or Pools to make room for a 3 drop of any Island bound Fetchland you could get. Misty Rainforest would fit best but Scalding Tarn, Flooded Strand or Polluted Delat would also fit

Did some research and found what helps us to solve this problem here:
https://www.mtgsalvation.com/forums/magic-fundamentals/magic-rulings/783146-additional-cost-causes-ability-to-trigger-which
and here:
https://boardgames.stackexchange.com/questions/15009/what-happens-when-additional-costs-cant-ultimately-be-paid-but-the-spell-is-on

So what happens will be:
1 Placing EE on the stack
2 Determine casting cost
2.a Pay {1}{G}{G}
2.b Sacrifice WS
3 Put WS trigger on the stack
4 Pass priority

So yes, you are right because Ob Nixilis, the Fallen enters the battlefield after World Shaper's trigger resolved, you wont get any benefits from its Landfall trigger

No Constrictor?

Is it? Wouldnt it be the way that you just have to pay the cost completely before EE goes onto the stack? So WS trigger as part of the cost should stay first on the stack while EE for example could still be countered

Dark Ritual is not Modern legal

"basically you can only win with Goblin Charbelcher. Everything else is just there to make sure you can get it.

This probably sucks."

Use some storm spells like Empty the Warrens and your problem should be solved thought

I have made an example based on the Challenger Deck: Counter Surge; so it is some kind of budget thought
https://www.mtgvault.com/risendeep/decks/surge-control/

Dont play control with Merfolk in standard like that, Ixalan Merfolk is made for aggro more than it is for control. You have some cards that are good for control decks like Deeproot Champion, because Deeproot Champion gets a counter every time you play a non-creature spell for example. Kopala, Warden of Waves gives ya a creature with a 2/2 body that hinders spells to be used unless for an additional 2 or Swift Warden that gives Hexproof.

A control deck is more kind of keeping your opponent on the short leash while you prepare your win condition

Empty the Warrens or any other storm spell, problem solved!?

Only one Path to Exile seems very unproductive to me, would play 3 copies at least. When going White you could also use Sygg, River Guide. That small little ass saved mine for a lot of games against Jeskay Control and Living End

Seems pretty ok to me, experience comes while playing and the game just needs some brews because everyone copy meta decks dosent make the game fun ;)

Maybe anything with Explore ability like Jadelight Ranger could also fit into the deck

I think the one green source is included to support Flashback from Ancient Grudge only, not to power Tasigur. The other way round, why he (this is a copy of the list from Pro Tour Top 8) included 3 different fetchlands to red as a combined ammount of 4 instead of 4 copies of a single fetch land, I even dont know

Depends on the format; for someone like Legacy or Vintage you will find a lot of different cards from the Tutors up to Ancestral Vision. In modern you have some resources to use like Serum Visions, Opt, Sleight of Hand for simple card draw, Whir of Invention or Fabricate for targeted artifact search or Nissa, Stewar of Elements as an example that contains both, Scry 2 as +2 Trigger and then when it is powered enought the 0 trigger that lets you put land/creature cards into play for free.

I like Nissa very well but it is at least a 3 mana drop so you should consider using something with "more value" in early game, on late game she is unbeatable as search and/or spawn trigger.

Edit: Forgot about some cards with the Explore trigger. I fiddled arround with Jadelight Ranger and Gaea's Blessing a lot last times

I'm missing at least one important card for commander, Empress Galina while I see you already have Thada Adel, Aquisitor added. Both have there values while Galina can steal nearly anything legendary (since the rules changed even Planswalkers and your opponents commander) while Thada has two advantages; she first has Islandwalk on her own and second will grab any artifact from your opponents deck so "second Sol Ring anyone?".

You could also consider to add Kira, Great Glass-Spinner as an addition to Kopala. It may not be a Merfolk but also has its values.
A second good non-Merfolk supporter card I like to play is Patron Wizard. Since most Merfolk you play are Wizards too, they can be utilized to counter spells just for tapping them with Patron Wizard. Fallowsage for example profits from this while helping to counter it will also draw a card.

What happened to good old True-Name Nemesis?

A board finisher could also be Deepchannel Mentor because it makes any of your blue cards unblockable, he may get the final strike.

So now to some cards of your deck: Augur of Bolas seems a little misplaced here because you run just 11 spells it could target, what are 11% of your deck while it will in 89% of the cases throw away some cards that might have been important for you. If you like to run a filter creature, consider for another Forerunner of the Heralds card, Tidal Courier. Merfolk Wayfinder is the same for Island cards targeting any Island card just like Breeding Pool too.

For a bit more power you could also add Metallic Mimic as a second Deeproot Elite and while your commander is Kumena, also Sage of Fables. Sage also adds counters your your Wizards cards and can utilize those for drawing a card for each counter on target creature. Shapers of Nature's second ability costs one blue mana more but is also an alternative

Getting stuff into grave, just use Jadelight Ranger or any other explore card. I also would put Collective Brutality into main deck