UniversalMan

My 5 cents:

1. ASU Zeus: instead of or in addition to useless pure nuke damage power adds current levels of Static Field (amplifying damage, scales to late-game) and Lightning Bolt before ult (to prevent waste usage on invis heroes). If you don't have these 2 skills it uses lvl 1 of both skills.
2. Primal Split. Adds new Synergy effect (if you have this ability): increases HP pool of Earth Panda based on your STR and adds 2sec Taunt ability to attract your foes (no manacost, 5 sec cd, 200AoE); manapool of Storm Panda based on your INT and lowers cd of his skills by half; attackspeed of Fire Panda based on your AGI and makes attacks never miss his targets
3. Frozen Sigil. Adds new Synergy effect (if you have this ability): increases duration by 5/10/15/20 sec and doubles slowing effect
4. Healing Ward: change its HP from 5 to 1/2/3/4 hero attacks required to be destroyed per ward's level. Non-hero units do only 1/3 of damage. + Synergy effect: your ward becomes invulnerable for its duration.
5. Weavers Swarm + Synergy: increases armor reduction per attack and reduces cooldown between attacks.
6. Spider Sences Aura rework (Spiderling + Synergy): now it is a passive ability carried by Spiderlings and Spiderites: each attack increases attackspeed and damage by 2/3/4/5 for 3seconds and each spider within 900AoE improves this value by half as much. Stacks infinity and independently. Each spider in presence and each attack increase number of stacks. Spider Web counts as 10 spiders.
7. SD Disruption + Synergy: increases duration by 1/2/3/4 seconds and illusions movespeed and attackspeed by 5/10/15/20%
8. Netherward + Synergy: increases ward's damage per point of spent mana and adds Banish ability (like on Tombstone), but I suggest to reduce cooldown of Banish so that you could use it 2-3 times within Tombstone/Nether ward duration instead of only 1 time.
9. Manta Style + Synergy: illusions benefit (standart bonus 5/10/15/20% magic resistance increase)
10. Necrobook + Synergy: increases duration by 10/20/30/40 seconds and allows to have multiple couples of Necro units, also increases HP of units by 50/100/150/200.
11. Necrobook + ASU: Necro units no longer give any bounty or experience to the killer and become Wraiths for 7seconds (like Skeleton King ASU). These Necro units are visually different for both allies and enemies.
12. Hookshot ASU: now ignores any non-hero units on its path. So you always latch to any hero or your hook fails if it doesn't find anyone. You can still target any unit on cast.
13. Rocket Flare: now reveals also an area on its path to the destination that remains visible as long as the destination area. Duration = destination area duration + time since the rocket travels. Also increase rocket damage or reduce its cd to fire more often.
14. Duel ASU: no longer blocks any passive abilities from caster (still cannot use any actives, but can proc for instance bash or use cleave etc)
15. Homing Missile is invisible and invulnerable while it is inactive. Powershot is invisible until it hits the first target. AA's Ice Blast second projectille is visible to both allies and enemies as well as destination AoE.
16. Blink Strike blocks the target's turn rate for 1/1.5/2 seconds. Same as in Dota Imba.
17. Life Break self cost on ranged heroes is reduced by half.
18. Meat Hook drags the victim to the casting point rather than caster's current positon. This prevents the caster from TP to his base or swap with other unit increasing that way dragging distance.
19. Static Remnant and Fire Remnant can attack nearby random targets using casters attack range upon cast, current attackspeed and current damage. Static Remnant uses current lvl (or lvl 1 if not skilled) Overcharge each time its owner casts a spell. Static Remnant detonates only if at least 3 enemies are in range. If life duration runs out it still detonates but also uses Overcharge. This idea came also from Dota Imba with some modifications. Also to make leveling this skill more worthful increase its duration to 12/16/20/24 seconds, cd to 4/3.25/2.5/1.75 and manacost constant 100 at all levels. Fire Remnant: if life duration runs out it still detonates an area for damage and if its owner takes more than 300/400/500/600 damage in one instance the Fire Remnant engulfs itself with Flame Guard of equal level (the owner does not need to have this skill). If the owner wears Flame Guard and if not on cooldown the Fire Remnant also wears Flame Guard. Doesn't stack with itself and cooldown cannot be refreshed by any means on Fire Remnant.
20. Greater Bash and Empowering Haste fully depend on current speed, i.e. if owner has more than standart 522ms via triggers it benefits from bonus speed. Same goes for Nether Strike and Charge of Darkness. Currents skills are all useless unless you use combo of them.
21. BKB components. Current: Mithril Hammer (1600) + Ogre Axe (1000)  + recipe (1375). Suggest: Ogre Axe (1000)  + Vitality Booster (1100) + recipe (1875) . Same price but more useful components. Undroppable but charges can be refreshed by buying the recipe again.
22. Radiance Aura. The afflicted target no longer miss from Radiance Aura if he attacks the Radiance wielder. For balance reasons.
23. Divine Rapier. Original Rapier mode always, i.e. droppable upon death from anyone, unsharerable, benefits only the buyer or the first opponent who picks it. Can be carried by couriers but drops if the courier is killed (unlike other items). 2 Divine Rapier into one item Epic Rapier.  Provides + 1000 damage. Original Rapier mode until the buyer dies. Free Rapier once picked up by enemy. Cannot be picked up by any courier in any mode.

AFAIK, it has been designed so in original Dota since last patches (around 6.80~).

Also Marksmanship works only on ranged AGI heroes. On anything else it does nothing, just wasting skill.

ASU Grow! doesn't work on ranged heroes. It only provides 107 bonus range, but doesn't give cleaving attack and demolish against structures. I think it is intended but the desription says nothing about it. So you should write a warning about it. Else change the ASU Grow! skill and balance it.

Is it intended or not that practically none of the spells and abilities checks whether the target is mechanical unit or not? I know the only exception with Heartstoppers. I can cast Fireblast, Frost Nova etc on Meat Wagons and they take damage and side effects like normal units. Especially annoying with Radiance.