He's in a similar position as Joshua and Whyte in terms of his career and has no upcoming fights booked. Joshua is lower risk than Wilder, who he has only teased about. It's lower risk for Joshua than Hrgovic, could draw a big gate considering it would be a rubber match. It's kind of the only possible name that excites me. I wasn't even going to watch the Whyte fight but I would watch him against Ruiz.
a) Different style (opposite spectrum of hand-speed; multiple subtle counterpunching set ups versus fundamental boxing with some cheap tricks fixed in; amateur pedigree with great balance and composure versus uppercut magnet, often off-balance, careless load stepping). Joshua and any reasonable handler at this delicate stage of his career would want a dedicated camp. b) Ruiz is dangerous, likely in training for a huge fight (negotiating with Fury and Wilder) and motivated since he is aiming for the last big chance of his career. c) Ruiz fight would be a trilogy, versus a Mexican (the only heavyweight champion in their history to boot.) You do not waste such fight without milking it to the last drop with location and promotion.
Let's go Joshua, take the fight and get respect again, don't let your fans down and the other boxing fans who paid in advance to see you on August 12th.