To be grammatically correct with regard to use of prepositions, you should reword the phrase:
I would like to propose forming a partnership where we work together to provide optimal service to the new developments you are building, or those on which you have already started construction.
However, the last two phrases seem redundant. If they are building a development, it's understood they have started construction on it. As such, the end can be omitted:
I would like to propose forming a partnership where we work together to provide optimal service to the new developments you are building.
In addition, I would tweak it a bit further to make it flow better:
I would like to propose a partnership wherein we work together to provide optimal service to the new developments you are building.
It's understood that the partnership is between you and the other party if the goal is that you would be working together for a common goal.