When building my house in CT, return air ducts are not put into basements because of radon, at least along the coast where radon levels are high. In prepaing for an addition to the house, I will be adding radon remediation. I don't like the pressure differences in the house currently as the basement has supply ducts but no return ducts, and feel that adding returns will help with better more consistent temperatures throughout the house. I'll also be adding an ERV to the current HVAC system during the addition process. I'm curious if anyone has done this process and what the results were.
As you certainly know, omitting returns in the basement isn't exactly a solution for radon :). Preparation for radon in new homes is an absolute must for people in locations like yours and mine, problem areas. Any new home built without such could face a huge penalty when it comes time to sell. Any major renovation like what your planning is a good time to play catch-up.
I still remember the first home in our neighborhood that failed a radon test, at least the first I heard about. I think a haunted house would have been easier to sell. As time has passed, that fear has switched places with one of not having the remediation in place, now that everyone has accepted that the radon is always going to be there and needs to be taken care of.
Back to your house, if you do a great job of remediation, I would not be worried about the heating system circulating basement air into the house. If the remaining level is higher than you would like, then isolating the basement with its own HRV and heat is possible.
Your first step is the remediation and a look at those results.
As for the consistent temperatures throughout the house, that is a process of delivering the right amount of heat to the right places and you are correct that return air is a big part of that.
A thorough sealing of the duct system, including the furnace cabinet should do a lot to remove pressure differences from the air handler. Radon issues aside, there are a number of reasons to avoid basement returns.
Greg, what are the numerous reasons you mention?