The mercenary cost depends on the current equipment and level. I remember ressurecting my rogue companion in act I for like 90 gold.
The evasion doesn't depend on anything other than defense. Tbh the defense in Diablo II hardly makes sense, it should add damage reduction rather than stacking evasion. Although there are heaps of items that grant physical/magical damage reduction, along with the percentage variation.
Well that kinda sucks. The only abilities for Amazon that works with any weapon are Passive and Magic skills, but they are generally used to gain better mobility and hardly factor to the fun in the game.
You could put several different skiltrees in different spellbooks and give the user absolute freedom of picking any spell, that's what Diablo II does. When the user has the wrong weapon equiped and tries to cast a spell that requires a different weapon, simply do nothing or return a SimError that says "I can't use that". Actually a more fun implementation would be a sound that says that, taken from Diablo II itself, that would definitely factor in the general Diablo II feel.
Since I've been talking about skilltrees and spellbooks, I suggest you have 4 different ones. Namely: Passive and Magic skills that work with any weapon, Javelin skills that work with javelins, Spear skills that work with spears and Bow skills that work with bows. Now to get to their key abilities, what makes their skilltrees unique:
• Passive and Magic skills should have Critical Strike and some sort of Dodge implementation. Not sure how would dodge interract with evasion, but I guess there could be plenty of workarounds. Later in the game it should have Valkyrie (a permanent companion that mainly acts as a tank). I don't know if you would get to that point of using a Valkyrie since it's level 30 and that level is reached at the end of Act V. Or you could just make it available at a lower level.
• Javelin skills should have Plague Javelin and Lightning Fury. Plague javelin throws a javelin towards the targeted angle, travels X range and leaves poison clouds in the trail. Lightning Fury throws a javelin, which splits into many pieces upon impact, depending on how many enemies there are around. Of course there can be variations that would lead up to these abilities because both of them are achieved on a high level.
• Spear skills should have Jab at first (triple attack as I said in my previous VM) then lightning strike attacks that upon impact would create a lightning effect - 1. Would release lightning bolts into the opposite angle of (Angle between caster and target) and 2. Would release a chain lightning upon impact. There can be Fend later on, that attacks all adjacent enemies, but I hardly use that because it's pretty boring
.
• Bow skills would branch off into 3 paths - Physical, Fire and Ice damage. Phsyical arrows would either be multiple just as they are released or just grant some bonus physical damage. Fire arrows would explode (later in the game they would set the ground where it was impacted on fire). Ice arrows would just freeze the enemy (later in the game it would have an AoE effect).
When I got to write about the spells I came across different variations of basically the same spell concept, for example the Fire Arrow type skills. There are three Fire Arrow skills in Diablo II - Fire Arrow/Explosive Arrow/Immolation Arrow. Fire Arrow would just do bonus fire damage, explosive arrow would do the same, just AoE, and Immolation Arrow, would do the same as explosive arrow and in addition to it, would set the ground on fire. I've been thinking about an interesting spell concept known as "Tier bonuses" which was implemented in Torchlight II. Basically what it would give, is a bonus to the ability every X levels. It can be easily done with all skills, whether it would grant bonus damage, new mechanics or just better manacost/cooldown.
Also, some spells of Diablo II are already made in hive, I don't actually know which, but BPower is a big fan of it and he made some spells, you could take examples
.
Cheers!
EDIT: There's a typo in your sig, it should say "Map description templates" XD