Don't know about attribute system like Warcraft 3 (I played a demo of it... twice), but there is no doubt a very good attribute system in BFME. By granting certain upgrades with triggers, there's no issues with displaying the increased attributes from those upgrades (like unit Upgrades are done in vanilla). The problem would be if you can't merge them, so you have like +100 health 5 times instead of a combined +500 health in the tooltip for the hero. It can get a bit overkill with tons of text. By simplifying the rank system, you can easily set the different kinds of enemies to grant, say, 1, 2, 3, etc.. XP to the hero (who needs 10 XP for rank 2, 25 XP for rank 3, 50 XP for rank 4, etc..).
At each rank you could get an upgrade, which boost whatever. In addition, if you want choices between speed, armour or damage for instance when you rank up, you could have radial buttons appear when you rank up (ie, hidden when upgrade not met), and make them conflicted. You choose one, and can't get the other ones ever (they hides afterwards).
Special enemy units could grant special upgrades when destroyed to the team that did it via map scripts. Same for special chests that can be destroyed; they act as an enemy you have to defeat (unlock). When you do, you get an item. You could even have a random item be given. The item would of course be an upgrade that gives the hero a new sword for example, if new models are allowed (hell, CaH models might be perfect for the job). It could also be possible to change the weapon with a sub-commandset with different weaponsets to choose from (when discovered. I think it's possible to hide the possible pre-determined sets too, so they would all appear as "???" as long as you haven't found the "item" (without changing the commandset completely). Not quite sure though..)
The odd exception is mana. Here you would need to use map scripts I think. How would it work? Depletable? In that case, no cooldown (or whatever's normal), but use counters in the scripts. Everytime a hero uses a spell it adds 1 to the counter (say, max 10). So you only have 10 times to use spells. If you find a mana potion, you could decrease the counter by 2 (or increase the limit by 2). When the counter hit's 10, the special powers are all disabled (they would require the no-go tooltip).
All that said, MOBA's aren't something I play or fancy, so I could understand you wrong. But I believe that it could work very well indeed. I didn't feel that this project would be worth anything to me, but after having thought about all of this I do feel that it could be very cool to test the limits of the game. Damn you!
Other than that, take a look at Grim's. Can't recall it all, but some ideas could be dug up?
Edited by Lauri, 07 April 2013 - 01:29 AM.