Using alt+e doesn’t create a wall under tassadar. not sure if this is intended, but it feels like it should be an option.
It’s question of vector targeting which prevents this from working as you believe it should. Xul’s Skeletal Mages ult won’t cast in the same fashion either and there is at least several other examples of similar behaviour in the hero pool. I think it includes Whitemane’s W ribbon self cast command as well which prevents her from casting it on top of her position - maybe intended since it wouldn’t be able to hit an enemy hitbox with both lines of the cross if the cast were centered on her if she stayed immobile.
This sounds more like a technical limitation in the engine rather than a possible quality life improvement like everyone would want it ideally to behave.
Coincidentally allowing Tassadar to spawn the wall on top of himself could result in him being pushed out in unwanted manner, similar as users have been reporting the behaviour of Leoric’s wall and heroes randomly escaping the enclosure.
I believe hots developer intuition is decently high to predict any problems that could arise by changing some game world rules.