When AoE III came out, it was quite hardware-intensive for it's time. Visually, it looked great, but it was such a departure from AoK that it felt like a completely different series.
I think the 3-D aspect added a new layer of complexity. That, coupled with the seemingly much closer zoom, made things feel very different. Don't get me wrong, I had many hours clocked with AoE III and it was fun in its own ways. But for AoE IV I hope Relic doesn't make it too graphically intensive.
I would rather have AoE IV have huge maps and the ability to zoom out far, rather than have bleeding-edge visuals. Something along the lines of the textural detail of AoE: DE, but with the realistic physics / animations of AoE III, would be great.
Unit count probably factors into this as well. Realistically 500 is the absolute max that I think is manageable, but unit management has to be improved and optimized for huge armies (i.e. see my earlier post about combat realism). But with 2018 graphics the game would have to be very efficient.