I have two empty neutral slots because that's how many I could fit evenly into 3 rows of 9. I don't have any intention of adding any new classes at least until I completely finish everyone (mainly Yuna). When that time comes, I have tons of room to add new classes (notice that I left a lot of area on the screen).
The problem with many MOBA classes is that their skills don't tend to transfer well into this mod.
I've said it before somewhere that I would really dislike traditional-style stuns (can't attack or move during), because they take the fun away from the guy getting stunned. Normally this isn't as big of a problem in MOBA games, because you can just build up items for more health, armor, anti-cc, etc., but in INSmod, you don't have that option. I've contemplated this decision a long time ago when I first added Dante (though about Cerberus crystal attack).
Most individual classes kill so quickly that anything that disables would pretty much be a free kill. Mobility is such an important part of the gg2 metagame. Imagine how bullshit a windrunner shackleshot into a tassadar psionic storm would be.
Besides that, I do take a lot of inspiration from certain MOBA hero skills. More things with positional mechanics, unique attacks, buffs and debuffs. I'll steer clear from percentage based things (critical hits, dodge chance, chance to proc, etc), most disables, and silence.