So I've always assumed Mars never had enough available CO2 on-site for terraforming, something that new research seems to have confirmed.
But my ideas have always relied on getting CO2 from alternate sources, e.g controlled impacts or in this case borrowing gas from Venus. The atmosphere of Venus is about 90 bars of pressure, and we'd only need around one (or more?) to bring Mars to a livable pressure. Transporting that gas however is a bit of a pickle.
One idea I've had is using a fleet of Venus-Earth-Mars cyclers to transport pressurized Venusian gas (perhaps mostly unfiltered as Mars can make use of the nitrogen and trace elements too) to Mars. As an added bonus, the canisters can be created from on-site carbon and burn up on Martian entry. I'll opt for the cargo capacity of the average oceanic cargo ship of 25,000 tons, or a little under 23 million kilograms. In terms of mass, I'm not sure how much gas is required to bring Mars to 1 bar of atmospheric pressure. I assume it would be slightly more than Earth however due to the lower gravity. Earth's atmosphere has a mass of about 5.15 * 10^18 kg. To make things simpler, I'll go with that. It would take one of our cyclers 2.239 × 10^11 trips to complete its objective, but a fleet of 100,000 would bring that to a more modest number...at 2.239 million trips.
This is where my already poor math starts to fail: I can't calculate gravity assists and I don't know how long it would take a Venus-Earth-Mars cycler to complete a round trip. So I'll REALLY fudge it from here and just put in the time of an Aldrin Earth-Mars cycler, 779.27 days, and add an additional 200 days for the ~100 days it takes for the average spacecraft to reach Venus. ~2.683 years * 2.239 million trips means Mars would have 1 bar of atmosphere in...6,007,200 years. Even with an accurate round-trip time for the cycler, that's still a 'yikes' from me. By the way, it would take only 300 years for 2 billion 25,000 ton ships to transfer the gas...I'm sure there is a much better use of interplanetary resources, probably.
Another idea I've recently had is a mass driver (or a 'space hose'?) that would constantly fire Venusian gas towards Mars. It would be an orbital (either conventional or as an orbital-ring space elevator) or very high altitude platform (possible with helium aerostats and solar powered turbofans) that would fire gas either in the form of the pressurized containers mentioned earlier (good for in-atmosphere installations) or straight up unprotected gas straight from the nozzle, a high-velocity narrow jet being fired straight into space.
From there I have no idea where to start...I don't even have my ideas for the 'space hose' solidified just yet. Still, I wonder how long it would take an array of mass drivers to terraform Mars, and importantly would the naked gas in Venus-Mars transit have issues with drifting away from solar influence?
If anyone would like to offer math of their own, or their own ideas on how to transport the gas, feel free!