1. Color change rate.
This should be increased somewhat. It is not THAT hard to go get another colorful key item if you need to pop T2s for magians. It is much worse to have this low change rate for people who need T3s. Not to mention you only need to kill like 6 T2s, while you need to kill 5-10 T3s for the drops.
2. Repop rate.
I hear Abyssea NMs have longer repop than normal at T2+. Considering regular VNM are in 4 zones and 2 in each, but abyssea VNM is a single zone and monster with time restrictions, I think it would be more fair with 15-20 min repop times. Sure you can enter, kill it, leave, wait, and redo it much later and not waste too much of your abyssea time. But it makes it very hard to get any results in reasonable time. Alternatively make 4 pops per zone, so that you can kill just as many as in regular zones.
I'll probably complain about the useless drop rate if I ever get to killing an abyssea T3 VNM. You'll probably hear from me in about 5 months when it happens.