An intersection with a few LED signs would certainly give off a metropolis vibe similar to Dundas. 4th street SW and 17th ave would be a great location as well as 8th street SW and 17th ave around Tompkins (as previously mentioned) On a smaller scale, maybe Kensington Road and 10th street - if the Osteria parcel gets redeveloped.
I think that the Beltline is far too residential in nature to be a good place for a digital mini Younge/Dundas type square. We could borrow some small elements for a Tompkins Park redo, but otherwise most of the neighbourhood lacks monumentality that is required for these type of grand public spaces. It should be noted that these squares are only partially "invented" - mostly they exist, and are successful as tourist traps or otherwise, because they have always existed in some capacity at the crossroads of major commercial, retail and transport hubs (and at the centre of a large local population for more people to see signs and generate that all-important commerce).
So 7th Avenue / Stephen Avenue is the right vibe for "monumentality" - unfortunately the area is now painfully being held back because of our 50 year obsession of single-use office block development in the core that limits the 24 hour nature of the area. In fact it probably needs even less plazas than it has - I'd trade most of the corporate plazas in the core for a major movie theatre development (and the huge amount of traffic it generates) connected directly to a train station. For example, the city would be a better place if instead of the Courthouse Park at 4 Street SW Station, we built a 16-screen cineplex.
Back to Tomkins Park, it's a great local node and commercial hub, has a high (and growing) local population, as well as possibly the only area approaching 18 - 24 hours/day of vibrant street activity in the city. All big strengths. But it lack the monumentality of a Yonge-Dundas and the rapid transit connections. Given it's size and I would argue functions incredibly well currently.
For the non-locals (or those more familiar with the more discussed night-economy of 17th Avenue), tons of seniors frequent the park all morning and throughout the day, people have coffee and lunch, groups meet up there. It's not always posh people or fancy people going to expensive stores, it's a diverse crowd of locals and visitors. From Calgary's limited urban public space examples, it's got to be one of the strongest for a well-balanced, 18 - 24 hour public space. Of course, it could certainly benefit from a bit of a clean up on some sight lines, add some more seating and make the paths wider and easier to use - but it's success deserves more of a refresh, not a redo to a mini-Younge/Dundas style.