The feasibility study commissioned by the Maribor city council for building a tunnel under the old city is positive. Per IRGO Consulting study results, the 980 meter long tunnel is quite feasible with respect to geology and environmental concerns,

It is even more interesting I think that the municipality considers the initial tunnel under the old city can also be extended in the east direction to Mlinska ulica and Hitra.There will be a separate feasibility study for that.

From what I read, my feeling is the old city tunnel construction will make into plans soon.

https://siol.net/novice/slovenija/pod-srediscem-maribora-je-mogoce-zgraditi-predor-foto-538150