Extra ideas:
1. Use extra points to increase dmg/defense/health
2. Add extra items slots to further expand variations
3. Add ability to reroll stat on an item at a coin cost.
4. add 5th upgrade of items (Artifact instead of Artefact) with a 5% chance
5. Add ability to combine 3 artifacts into 1 more powerful (3 * x -> x+1, 3 * x+1 -> x+2 and so on)
as lower tier towers do a negligible damage consider to enable and tower slot update where you will be able to place higher tier tower. For example. In the T1 slot, 1st update will enable T2 towers. Cost will make sense to be higher than the cost of unlocking the last T2 slot.
 "War Lands"){kind=icon}
 "Funny farm"){kind=icon}
 "Idle Portal Guardian"){kind=icon}
 "Defender Idle"){kind=icon}
 "Idle Breakout"){kind=icon}
 "SAGA Idle"){kind=icon}
They're spawning in the adventure aswell. I guess you're a little unlucky :)