So I'm going to kick things off with one of the themes I love about the Primeval Thule setting - how magic is rare, dark and dangerous.
Unlike most DnD settings, Primeval Thule makes spell casters very rare, with not more than a handful in the largest of cities (of which there are few). For me this is a a very welcome change to the usual wide spread magic traditional to DnD settings. Most commoners have never seen a spell or enchanted object. Magic items are unique, and not something easily bought or sold.
Rarity can be reflected in the frequency of spell scrolls and magic items which is well within DM purview. But I also want to avoid what I like to call "ritual spam", so my tweak to rituals is they take 1d4 hours to cast instead of 10 mins. In addition, at the end of ritual the caster must roll 1d20. On a result of 1 or 20, something extra has been invoked, and the Dark & Dangerous Magic Table is triggered (see below).
Dark & Dangerous
I really like how Thule paints sorcery as unnatural and something people mess with at their peril. But the mechanics themselves don't really reflect this. Sure, there are some madness rules which can touch on certain types of spell casting, but I want something more concrete that that. Something that makes magic inherently more dangerous any time it's used; not just to the caster, but to those around them as well.
So below is my table of magical side effects with a dark and dangerous feel. There is a definite Cthulu bent, some temporary mutation, and hopefully enough randomization to keep things fresh. I modeled the table on wild magic from the Player's Handbook, but kept the overall spread of effects much more neutral (the wild magic table is heavily skewed towards beneficial effects). I've included a small number of clearly beneficial effects and a small number of clearly adverse effects. But the vast majority are simply dangerous effects, with about the same chance of being applied to the caster as the enemy. I have purposely avoided any kind of "nerfing" of casters (unless you consider the table's randomness an inherent nerf).
I recommend requiring a roll on the Dark & Dangerous Magic Table any time a spell is cast and the first 1d20 related roll comes up a 1 or a 20. If that chance is too high for your taste, you could limit the checks to 1st level spells or higher only (ie ignoring cantrips), or reduce the chance to 1 in 20 only.
So here it is, Houserule #1: Dark & Dangerous Magic. I hope it helps make magic a little more scary across the primeval continent.
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