To be clear, I'm not talking about creating human-livable conditions, just the minimum required to support microbial life and possibly begin the process of creating an environment to support more complex thousands-to-millions of years down the line. (Though maybe that's a severe underestimate, given that it took 3 billion years for Earth to get conditions to go from simple cells to bacteria)
Is it better to break down the question of "terraforming Mars" to creating conditions for life in only small portions of Mars? (Or expand living environment for existing microbes, if we're still holding out hope)
Hellas basin- What are the requirements for raising the atmospheric pressure at the bottom of the basin so that it is above the triple point year-round instead of just in winter? IE, would heavier gasses (Chlorine, etc.) pool in the basin and increase pressure, or would they end up dissipating too quickly to be helpful for supporting life? Would having a larger window of time where water can form in a liquid state be an enabling factor for supporting life on Mars? And I suppose, more practically- how far out are we from sending a rover or sensor to the bottom of the Hellas basin? It's been a candidate for rover landings, but I think it's been voted out every time due to issues with having connectivity/solar power. I don't think any landing candidates have been at the 7km deep location yet (The one that sometimes has appropriate conditions for liquid water).
Underground- How deep would we need to go to find an environment to support microbes that survive on chemosynthesis? IE, would we need to go deeper than on Earth to find a source of underground water/good temperature? Would introducing these eventually create conditions that support surface life (Even billions of years down the line), or is that too complex to predict?
Are there other places on Mars that might be "candidates" for life?