Transparency of WMS is very very nice feature but...
1. It would possible to change order of map i.e. online/offline vector/raster map over WMS with (of course) transparency slider.
2. I back to my idea (not my to be precise but implemented in OsmAnd): it`s possble to use to vector/raster map from which upper one is transparent. So we could use Orto map under transparent topo or vice versa.