Dream halls would be great fun to play, but, at this time, difficult to build.
I have some thoughts about it.

1.high tide/time spiral/turnabout. It's an engine for a storm count in combination with dream halls. Dream halls is optional, so the creation of a lot of mana could break the symmetrical effect.
2.disrupt for cantrip disruption. Holds the card advantage. Same goes for mystic remoera.
3.raven's crime instead of mana severance. Turns the dead draws of land into discard. Could be nice enough to run 2 or 3 copies of it. Dream halls is not so symmetrical if oppo has no cards.

thanks for your thoughts.

I'll tell you my considerations.

1. the three off-colour moxes ehlp me out to play turn 1 silvergill turn 2 reejerey. I agree on the mana vault.
2. FOW's have 23 other blue cards that should do the trick.
3. the untap/tap ability is so versatile that it tends to lure you to a combo style approach. Feels a bit like elves/staff of domination. You are right when you say that my first problem is to chose between aggro, control and combo.

conclusion is indded to test it in different styles and environments.

Some interesting cards to consider are static orb and rystic study.

I read your remarks, Mr.Burntqerbill. They are very helpful. I think something like this could work.

disruption

4 thoughtseize
3 cabal therapy
4 FOW

creatures

4 silvergill adept
4 merfolk traders
4 merrow reejerey
4 grimoire thief

toolbox

1 vampiric tutor
1 demonic tutor
1 diabolic intent
1 yawghmoth's will
4 brainstorm
1 echoing truth
1 diabolic edict
1 ancestral recall
1 timetwister

fuel

1 black lotus
5 moxen
1 mana vault
1 tolarian academy
4 polluted delta
4 underground sea
3 island
2 swamp

I think this would be a good starting point. The disruption of thoughtseize and cabal therapy, combined with Force of will, can buy you enough time to get the merfolks started. you have 4 adepts,4 traders, ancestral, timetwister, 4 brainstorms and three tutors to fuel your hand. That should do the trick.
In the sideboard you can have planar voids, pitching needles and more of that stuff.
every comment is welcome.

The tournament(with 5 proxies) was in december, so I have to think about yout questions.

My first matchup was against TEPS. I lost 1-2. I won game two. He mulled to 4 and I beat the hell out of him with a piferer. He never recovered. In game 3, we tossed some turns. He was disrupting and looking for keycards and I was attacking with a sivergill and a reejerey. On turn 4, I played tolarian and a darting merfolk. I asked for the stormcount of that turn and that kind of upset him. I had a vampiric,but no draw. the next turn he got off with necropotence.

Second matchup was against black-red aggro. I lost 1-2. A matter of the right draws. It could have been the other way around.

match three was against green aggro. I won 2-1, thanks to the reejerey and the extra cards drawn with silvergill.

match 4 was against green-red aggro-land destruction. I lost 0-2. His mazes of ith and his pyrostatic pillars proved their value.

Match 5 was against goblins. Me deck did well, but I lost game three.

I was fairly content with my new deck but I think I need some adjustments.

1.Replace the pilferes with merfolk traders. The pilferers did nothing against aggro. The card drawing and selection of cards is more important in this deck.
2.Putt in merchant scrolls and add 1 copy of mask of the mimic. The merchant scroll I can use to open my instant toolbox(echoing truth,FOW,stifle,maybe high tide, ancestrall and mask of the mimic) I think mask of the mimic is worth playtesting. If I need an extra reejerey or an extra silvergill, I can go fetch it.
3.Apathy in the sideboerd against aggro. Means card advantage if he wants to use his creature.
4.Grimoire thieves might be handy.

I played a merfolk deck  in a vintage tournament .

I started with:

4 silvergill adept
4 merrow reejerey
4 riptide pilferer
2 lord of atlantis
1 darting merfolk

and added
4 duress
4 force of will
1 mana vault
1 mana crypt
1 tolarian academy
1 demonic tutor
1 vampiric tutor
1 time twister
1 black lotus
1 yawghmot's will
2 sword of fire and ice
4 brainstorm
and a few other spells

It preformed quit well. Just 2 matches were lost without a real fight. It needs some more focus. I added tendrills for the chance of a darting merfolk+merrow reejerey+ tolarian academy.
And I replaced the pilferers for 4 merfolk traders.