The depth of the character progression system is somewhat shallow and players exhaust the possible strategies and strategic sidelines for their heroes relatively quickly. Luckily there are a lot of heroes and even after getting somewhat bored on one, you can switch to another one. However, this could be improved upon by expanding the item list to allow making certain decisions that currently are not available, as well as differentiating the currently existing items from one another somewhat further.

Dominated strategies and overlap
Due to various reasons there may be a specific choice of item for damage output, that on some heroes always perform better than another. On the flip side of the coin; sometimes the items are too similar in their effects, so that it ultimately makes very little difference which one is chosen, although for a highly competitive game even gaining small percentile advantages can be meaningful.

List of things items (and abilities) could provide in various additional ways
1. Healing and countering healing
There could be some early game items which would provide single target heals, and there could be additional items to counter non-regeneration based healing. These would amplify a healer role for early game.
2. Evasion and countering evasion
There could be some early game sources for evasion, as well as counter strategies for evasion. On this note an additional item to upgrade band of elvenskin into could be a choice?
3. Armor countering items
An item which would be most cost-effective when used against opponents of high armour, and least cost effective when used against opponents of low armour. Also I think ring of protection, buckler, chainmail and platemail could have another upgrade path(s).
4. Specific counter for hex
5. Another source of spell amplification
6. Items that would specifically reduce damage from attacks based on whether they're melee or ranged. (Property embedded into some other item perhaps)
7. An item that would counter lifesteal (slightly) against the user effectively (but would not affect lifesteal from allies in anyway). Could also be an ability on a hero.

Broader list of choices and intermediate items
To enable strategy in early game

Graph representation of item system for beginners
The selection of an item displays the edges and neighbouring nodes. Viewing the item list as a graph could help newbies get started faster.