Then you should try an play him first before posting some idea's.
All summons are based on the Summoners intelligence, so the more intelligence the stronger the summons.
Meat Golem functions as a tank, he gets 2x Summoners int as agility.
Pure Death functions as pure dps, however he barely scratches creeps.
Hell Hounds are a bit tank/dps hybrid.

So most of your suggestions aren't suggestions at all because it is already how the Summoner works..
So again, play him before u post it!

Anyway, the hounds need a small critical, and Death also needs a critical. I mean seriously...I have Legion Ring which for me takes out around 1/5 or 1/6 of Thanatos's health. Without using the spell, it takes me 8-10 minutes to kill Thanatos. I realize I'm only 832 so far, but still! An MR or PR could do this FAR faster, so could an elementalist!

Dark Summoner has become a tank (from summons) and has the crappiest DPS ever! He can train alone sometimes, but eventually fails.

(PS: Summoning Improvement needs more levels, it's nearly rigged right when you get it, 200-250 level's later it's basically useless. It's armor/regen for Death and damage for Golem, and a lesser effect of both for Hounds, which only deal damage from their numbers, but any chaotic boss can kill them easily.)

Have the summoner having a scaling spell that summons an item (only usable by summons or just summoner) that is unique and gives the summons an ability either while summoner is holding it or the summons. If it was the summons had it it would be like summoner summons hounds , then casts the item spell, all summoned hounds received xxxxx granting xxx xxx xxx. or the summoner is holding xxx granting all summons xxxx. allowing him to summon multiple items but only allowed to chose one of the summoned items for his summons to gain something special. (yes the second option means the summoner must have 1 open slot every game to benefit from it, and yes it would mean more coding for random items and needing more items specifically for this).

Or
a summoner spell that allows the summoner to activate a random ability(s) every xx giving all 3 summons (and future summons) a pure random ability from a set number of xx spells. after xx the ability(s) on the summons disappears and the summoner must recast to give new ability(s).