owned
Excellent OT User
The customization is of Ownership.
I imagine a veteran player will own their own house (which you can build after buying a plot of land) with pets, and NPC family and/or maid and guards or servents, and a wardrobe filled with armors and weapons and rings and amulets that they have leveled up and customized. Along with a Rune Making Machine that you put in recipes and it will make runes (Similar to smelting iron in minecraft) so that every day when you log in, waking up from your bed, you can pick up what you have, and go collect more. Increasing your net-wealth.
Now if you want to know about actual combat customization.
Each weapon, shield, boots, ring, spellbook, or wand can have a huge number of spells or abilities on it.
For example, you could loot lets say a Sword can have Berserk on it, and a Wand of Vortex can have Heavy Magic Missile, a shield can have barrier, along with their attackspeed, passives, minAttack, maxAttack etc. And as you level up an item, you can increase the spell's minPower, OR maxPower, OR (decrease) the cooldown, or the actual item's stats (defense, armor, minAttack, maxAttack, maxHealth, etc).
So as you play, you will probably have multiple choices for items you will wear in different situations, maybe a Bow that has a Paralyze every 3rd shot passive when fighting melee-oriented monsters. Or maybe a spell that knocks targets back, or stuns them, or makes walls, or freezes them, or fears them away, etc.
The fun of the server is collecting items with spells and passives on them that work well together and are fun to use. Think of it almost like Pokemon, where there are tons of these collectables out there, each having their own unique and fun playstyles, and you can level them up and customize them to suit your playstyle.
You're really no longer making a tibia server, you're designing your own game which is awesome. It'll be interesting to see the outcome of this if/when you finish!