If you find something, whether custom class archetype, feat, spell, creature to wild shape into, magic item, etc., and it sounds reasonable, I will probably let you have it, perhaps with some tweaking or additional in-game justification if necessary.
All spells in The Player’s Handbook or Elemental Evil Player’s Companion are allowed, and function as written, unless otherwise noted in the List of New or Changed Spells.
When you drop to zero hit points, you gain one level of exhaustion. The levels of exhaustion can be found on page 268 (pdf) or page 291 (book) of The Player’s Handbook.
Usually when you level up, your maximum number of hit points increases by an amount equal to one roll of your hit die plus your Constitution modifier. The house rule is this: when you roll your hit die, if the roll is lower than the average die roll, count it as the average rounded up instead. For example, if your hit die is a d8, a roll of 1, 2, 3, or 4 would all be counted as a 5 instead, while a roll of 5, 6, 7, or 8 would count as that number. This system keeps anyone from being crippled by low die rolls, but preserves some degree of randomness.
On a critical hit, if you roll less total damage than the maximum of the normal damage roll, you deal the maximum of the normal damage roll. This is so you don’t get a critical hit with below average damage. Example: if your normal damage for an attack is 1d8+4, the maximum normal damage roll is 12. If your critical hit of 2d8+4 is less than 12, you deal 12 damage.
On a critical failure, I will roll on this Critical Failure Table to see what else happens to you, aside from a normal miss. The distribution gives a roughly 30% chance of a normal miss, a 30% chance to provoke opportunity attacks from nearby enemies, a 20% chance of either giving you one round of disadvantage, or giving the enemy one round of advantage against you, and 20% chance of something worse happening.
Some higher level spells require material components with a monetary cost. If you find yourself trying to cast a spell you forgot to buy the components for, then I will let you roll a d20, call high or low, and if correct, you can retroactively buy the materials you need and cast the spell.
This campaign will feature a lot of traveling, mainly (I expect) by ship. As such, the PC’s are going to have a lot of downtime, but many of the usual downtime activities do not apply. Because all of you are spellcasters, and many of you have proficiency with the herbalism kit or alchemist’s supplies, you all have the potential ability to create magic items. The rules for doing so can be found in the Rules for Crafting Magic Items. The basics for crafting magic items mostly line up with the rules in the DMG, but have been sufficiently tweaked to warrant their own post.