I'm not a fan of term limits, but I was thinking of another solution:
Have fewer wards than what we currently have - something like 32 or 36. Small enough that there's access to a local councillor, but larger wards than currently.
Local ward councillors may only run for three terms. That's enough for a balance of institutional history and enough time (12 years) to get some projects through on one councillor's watch. At the end of those three terms, that ward councillor could run for a regional councillor position - 8 or 9 regions, or, at-large. Those positions have no term limits.
A very good city councillor could run for a promotion, and have a much larger area. The mayor's executive and committee chairs could then have to come from those regional (or at-large) councillors. The regional or city-wide council seats would be open to any candidate, but it would be a place for remarkable local politicians who are known outside their wards. It also forces the mayor to not snub parts of the city - something Lastman and Tory are notable for. (Miller had broad suburban representation in his inner circle.)