You put the bathroom anywhere you can install a proper drain. That drain by the furnace may or may not be of value for this job. A toilet must have at least a 3" line and be vented. A shower needs a 2" vented line. Sinks and tubs need only a 1-1/2" vented line. Now the problem you have to solve is how to provide a 1/4" per foot slope to your drain lines. The further away from the main drain you want the bathroom, the deeper your trench between the bathroom and main drain must be. It would be wise on your part to have a plumbing contractor evaluate what you have and advise you on what can and what can not be done. You will be covering your drain with concrete and it had better be right before you do that. Remember, all things are possible...if your pockets are deep enough.