What is the single most important aspect of developing a successful terminal plan?

Teamwork.  A successful terminal plan requires teamwork that takes into account the full spectrum of technical and operational issues relative to the primary facility stakeholders - the traveling public, the airlines, concessionaires, other tenants and the airport operator. .  It is the job of the terminal planning team to invite the participation of all stake-holders and to know how to synthesize that input into a plan, or group of alternatives, that answer the needs of the project.