Merfolks, as far as I know, have been kept mostly unchanged from pre-stone soup era outside of minor changes here and there. Considering that many species that were deemed redundant or uninteresting have been subjected to change or deletion, I think it is worth paying attention to current state of this watery, ancient folks.
Currently merfolks seems to be dex-oriented(due to its great ev&stlth and terrible armour skill), mostly versatile species which also are great at using polearms. Also they can kite enemies with spear in watery terrain.
This instantly gives me a warning sign, as kiting is and has been a playstyle crawl tries to get rid of. Furthermore I think any ability that is tied to something that player has little control over like terrain gives an unpleasant experience to player, as they cannot utilize their strongpoint with reliability.
I think the concept of dex oriented polearm wielding melee doesn’t work well since now LB became dex-based weapon. While MkGl with polearm is still viable, I feel like long blade are more suitable choich despite the aptitude difference. besides, if one is to create polearm-centric species, it should be done by giving them traits that synergize with reach weapon, not by artificially boosted aptitude.
Also, there already are enough species with dex-oriented playstyle that are acceptable melee / spellcaster / shapeshifer. I think merfolk needs some redesign if it were to fit in modern crawl.
My first suggestion is to rework them to be lore-centric. Fish people who use spear might have some appeal to people who enjoy folklore. Give them racial traits that synergizes well with reach weapon. They might splash water to shove enemies off of them. They might temporarily surrond them with deep water, avoiding direct contact with monster. Anything water-related that can be helpful to spear-wielder can work.
Another option would be to choose one niche that might be interesting and redesign it around that. For example, inspired by merfolk’s high shapeshifting and spear aptitude, warrior character that changes shape might be a concept worth pursuing. Currently many forms forces you to stick to unarmed combat; What if we can use specific weapon skill for unarmed attack? Mimic race that can change its limb into weapon might be worth experimenting.
Personally i would like to see modernized merfolk that feels unique while keeping their charm. However replacement will be also acceptable if it is for the sake of novel new species.