Some aspects to keep in mind when designing a last point for payload:
1. Finality - Does it feel final? Do all the routes in the map lead you this way? Obviously in alpha you won't need to worry about visuals, but is there a possibility for last to be visually striking? Do you want your final point to be more closed and fortified like Upward or Barnblitz, or more open like Badwater?
2. Routes - Badwater has 1 main route and 2 or 3 smaller routes. Barnblitz has 1 main route and 2 smaller ones. Borneo has 1 main and 1/2 smaller. Upward has 1 main and 2/3 smaller. In general, whatever path the cart goes down will be your main path, but don't forget to allow flanking classes like spy and pyro a way into the final.
3. Defense - Where in final would be a good sentry spot? Is it too overpowered? (remember, defending engineers have nearly unlimited metal near the respawn room) How close is final from red's respawn? Can red be spawncamped?
4. The Payload Factor - Remeber, payload is different from A/D in the fact that the focus moves gradually. Think about how the gameplay changes as the cart moves. Think about how defense will react to the cart being just after C versus just before D, and every point in between. The gameplay will always focus on the cart, so all the areas near the cart path need to be especially interesting.