Right now it is only possible to filter activities based on general type of activity (cycling, running, swimming, skiing). It is not possible to drill down further into these activities (e.g. show heatmap only for road bike). This would improve the heatmap feature a lot - as a road biker, I'm not interested in mountain bike activities. Even worse, sometimes you end up on a road which is not suitable for your bike. This could be easily avoided. I would even pay extra for this feature.