Here I am!
Old SK had one problem imo, and that was the fact that he didn't have any type of slow/disable which he needed just like people need air!
Reason for that is mostly because he was almost totally useless in early game, and some disable/slow would fix that.
Reflection fixes that and kinda sticks to the soul theme, but I do think it could use a twist or two (that Soul Split idea from the link provided by Monsterlord is a great start - maybe some mix of Reflection and Soul Split could be a winning combination).
So, in case he gets a remake in the future, I think he should keep this skill.
That aside, two things I loved the most about old SK were that he was a great HP manipulator (awesome concept which is pretty original and very effective if done well) and his reliable illusions.
His illusions were so sweet not because they were powerful (even though that did help
), but mainly because you didn't have to wait for cooldown and could always have at least one of them with you.
This means that he can almost never get caught of guard and doesn't need to wait for cooldowns, meaning that he can continue to stay active as long as he is alive or wants to - something NS and SK can't brag about!
On top of that, he had that sweet, sweet Soul Steal which would help him survive and would allow him to get his HP up really quick in case he needed to.
This combo made him quite a threat.
That did made him kinda mana heavy hero, but with good farm, that problem can be taken care of without big troubles.
His ultimate is also pretty damn awesome.
Only issue I have with it is range of the spell, but that's nothing a good ASU can't fix.
Last, but not least, there is Metamorphosis (I know I talked about the skills in a retarded order, but it simply came to me that way).
This skill used to take care of his remaining flaws; HP and range.
Now, even though current SK is kinda effective, he is more meh
For starters, he requires way less skill to be used.
He also lost a lot of originality (his illusions are just like Manta/NS/CK, but with Meta crammed in), which leads me to Zeal.
Not only this skill is simply bland and unoriginal, it is actually not even that good!
That HP regeneration helps at start while laning (but than again, so did Soul Steal) while IAS bonus is kinda unnecessary.
This is hero with already below average BAT and above average agility, meaning that AS and DMG are not that big of an issues for him.
Almost all items which are made for him also give ether agility, damage or IAS (ie. PT, S&Y, Manta, Skadi, Butterfly).
It's kinda like replacing Meat Hook with a passive which gives extra 300 HP or so.
Simply useless when compared to what he has lost.
Overall, old SK was not perfect and needed a remake.
Still, his current form is still crappy and he needs an another remake imo.
To sum it up...
Reflect could maybe use a tweak here and there (combining it with Soul Split from the link), but it's overall a pretty damn good skill and there are other priorities which should be thought about.
I think that Zeal needs to go since it's really redundant and having Soul Steal replace it would be really great.
His ultimate is fine as it is, but it could use an ASU to fix it's main issue - the range.
Other than that, it's a great skill.
Lastly, I think that somehow his old versions of Illusions and Metamorphosis should return.
I know that in total he would have 5 skills then (Reflection, Soul Steal, Conjure Illusions, Metamorphosis and Sunder), but I think that they can be crammed together into 4 without SK having to suffer (I know I kinda complained about current Meta being just two skills crammed together, but that's mainly because it was badly executed imo).
Well, it's 2:20am here, so some of these things could end up sounding wrong, but I'll come back tomorrow and will fix everything stupid.