When player upgrade commander to maximum level, he can't do anything with metal (only give to teammate).
I think will be cool if players can buy new units that will move with creps.
For example:
1 - Each wave go only basic creps.
2 - You buy "warrior"
3 - Each wave go basic creps and one warrior.
Units must cost much more.
For example: in normal game warrior cost 220, but in dota it must cost around 1000 (I think more).
With that think need some another changes:
1) Metall limit should be 4000
2) Creps should be weaker
3) Commanders should be stronger
Players buy new creps and in long game creps become real force.
p.s. I think technical part of the that idea no so difficult.