Between 1 and 4 lvls of skills
I suggest to set minimum difference between 1 and 4 lvls ~x2 times. Im talking about passive skills mainly:
1) Revert Blur change and MAYBE blink strike range.
2) Reduce spell shield back cast chance from 30% to 15...30% and mb resist bonus from 26...50% to 20...50%. It was so buffed in DotA at 1 lvl because AM is very thick and got reduced str.
3) Talon queen passive chance from 30 to 33% and dps from 30...45% to 20...44%. Yes, i know that it's not a high change, but makes better leveling. And still not bad at 1st.
4) Rescale ranged bashes chanse (Void and Slardar) from 5/10/10/15% to 6/9/12/15 or even to 9/11/13/15%. Reason — catastrofically low chance at 1st (no way to do 20 hits for 1 sec bash, defensive passives are much better, in many times) OR even made chance same as melee but lower bash duration to 0.6-0.8 sec.
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About changing Rubick's Spell Steal.
Same logic as in 1st part of topic.
Now it gives nearly nothing with leveling.
My version:
Cast range from 1000 (1400 agh) to 800/1000/1200 +200 with agh.
Cd from 18/16/14 (2 agh) to 16/13/10 (7/5/3 with agh)
Stealing spell lvl 2/3/4 for non-ultimates and 1/2/3 lvl for ultimates, based on spell steal lvl (same stealing Agh upgraded version if you have it)
Add passive component which increases spell power carrier by 3/5/7% to emphasize GrandMage's power.