Perhaps you go for the idea from shroud equivalent to other over times spells using spell regen points. This would also give our regen gear a new aspect. Dunno what atm our max possible regen is, players can reach.

But for summoning I would also restrict to skill level, would not be good if level 1 twinks ran around with army of skeletons. You can also attach summonings to charisma/intelligence stats, leading to higher mobs and more mobs.

Going in hp/mana/grace costs is also an idea, but atm our reservation looks not optimal on client. If you use rage, you always think you are injured.

Problem if you build spells with costs reducing their stats, this would lead more and more to food spaming.

If you think of this, you could also think of a necromancer class using there hp to summon monsters. Like they must pay for a golem with temporary life and some permanent life. This would open the market for pois in late game again.

Also interesting to combine this with alchemy, like you need grave yard dirt to summon skeletons. Or cast spells on corspe, for sure only neutral corpse.

So depends also on the spells, summoning skeletons, ghosts, golems or animate dead for example.

I've considered storms and forgot to mention an important point. As has been pointed out, players have no mechanism to enhance their summons. Attunement can help here but a binary value isn't a very good solution to anything. Player stats are a better solution here - we have 2 stats for each type of caster, which can be used to enhance summon HP (and/or resists) and damage.

Additionally I think a new weapon type could be in order. Currently scepters exist only as a "skin" for magic devices. We could move them to a weapon type of their own, which has minimal (or no) offensive stats but has stats for summons.

Well thinking logically I think life-linked summons have one potential issue (bug or feature is up to individual) that if you had let's say 10 summons and an opponent uses a storm attack then initial caster gets 11x storm damage assuming they're all hit with the same spell.

Yep, that's been a problem in my playtesting. That's part of why I think gear tied to summons is important - if players have easy access to some summon damage mitigation then being a summoner is a much more practical archetype.

1. Could summons have additional roles other than pure dmg. For example a healing summon that heals the user. In addition you could in theory have summons based around status effects. While more complex would a summon that loots for you be possible?

Both are easily possible. While we don't have AI functions to cast healing spells, it would be an easy feature to add. Auto-looting summons are also possible. Mobs have the ability to "investigate attractions", which means they move toward anything they see as attractive, be it a light source or an object of a specific arch or type. This brings the summon to the corpse, and an AI function can be added to search corpses beneath it and insert the contents into the owner's inventory.

2. Would upgrading your summon be possible. Would increasing it's HP, mana, defences, dmg, duration, cost etc be a good money sink.

Yes, and I think that generally these upgrades should be more available than existing gear upgrades. So a "scepter upgrade NPC" would be able to add these for a cost.

3. Would a dominate skill that converts a mob into a summon (while skill is in effect) be a good idea with success rate based on target's stats.

Possible, provided the right balance can be found.

4. How would a summon be obtained. Is it learning a spell/prayer, a random drop or using a special taming skill. Also if you do obtain a lvl 5 ant summon can you improve it or is it doomed to permanently be lvl 5 or can you in theory train it to 110 with either XP or resources.

Right now summons are spells/prayers. They use the player's Wizardry Spells or Divine Prayers level to upgrade, and kills with summons grant EXP in the skill that summoned them. Adding a summoning skill which acts like Wizardry Spells or Divine Prayers is possible, but would be quite a bit of work.

5. Could summons also have death sickness so it's stats reduce each time it dies

This could be possible, but monster stats (str/dex/etc.) serve different purposes than player stats so there would need to be an alternative death sickness which affects different stats for mobs.

Perhaps you go for the idea from shroud equivalent to other over times spells using spell regen points. This would also give our regen gear a new aspect. Dunno what atm our max possible regen is, players can reach.

But for summoning I would also restrict to skill level, would not be good if level 1 twinks ran around with army of skeletons. You can also attach summonings to charisma/intelligence stats, leading to higher mobs and more mobs.

...

Problem if you build spells with costs reducing their stats, this would lead more and more to food spaming.

Using a mana/grace degen could be a good alternative here. I don't think players currently have access to a significant amount of any regen yet, though. Using player stats could also determine summon level, if that's a better alternative to skill level.

Going in hp/mana/grace costs is also an idea, but atm our reservation looks not optimal on client. If you use rage, you always think you are injured.

This will be an easy fix. I'm not sure what the best appearance is, other than perhaps a mostly-transparent version of the HP bar which fills the reserved life region. This will require an update to 0.11 since the server doesn't send reservation to the client, but that's coming soon because all Windows clients will need a fresh reinstall anyway since the updater broke.

If you think of this, you could also think of a necromancer class using there hp to summon monsters. Like they must pay for a golem with temporary life and some permanent life. This would open the market for pois in late game again.

Also interesting to combine this with alchemy, like you need grave yard dirt to summon skeletons. Or cast spells on corspe, for sure only neutral corpse.

So depends also on the spells, summoning skeletons, ghosts, golems or animate dead for example.

Paying spell costs in permanent life loss is an interesting idea. Maybe an alternative version of our current caster skills could facilitate that.

Sorry for the double post but there is one potential flaw/feature with the normal summon logic. It can be argued that it is strategy so whether it's adjusted is up to you but I thought I should at least make you aware of it.

Currently any attack from a "normal" summon attracts aggro towards the caster. However if you end up in a map where there is a path 1 tile wide then you can end up with a situation where the summon is between the mob and the player with the mob taking hits from the summon yet it is unable to reach the player as path to the player is blocked by the summon. I believe a similar logic would also apply if a player goes through a portal with an example being bridge raas in countryside map. Potentially if you can summon enough summons if you can get up to 8 summons then you can surround a mob on all sides with mobs it won't target and then use spells to attack the trapped mob.

If you want a map where you can test it I'd suggest the Minotaur room in UC where there are 3 mobs that just use melee attacks and a long single file path heading to the stairs