It's a lot more profitable to have a 200 gate terminal than leasing gates, a 200 terminal can lower your gate payments at that airport by almost half, depending on the airport. For example, in S3A, I have a 200 terminal at ATH, and I only pay 152k for each gate compared to 217k for leased ones. Plus, your own gates cant be seized from the airport because of usage. Overall, 200 gate terminals are most profitable but it depends on the airport.
Just put your mind and think, play around with different designs until you find what you like the most, try looking at multiple liveries and putting your favorites together, ask some of your friends for feedback, etc. I usually come up with a simple design, play around with it, show it to some of my friends who might be online at the time of making, ask them for opinions and if they feel something should be changed, if they think something should be changed I'll change it and show it to them afterwards. In one word, it's just imagination.