I definitely agree that we should enhance the functionalities of warriors and perhaps greater demons.
I also wish that warriors were more interesting by default, rather than having to upgrade them. I believe that using your warriors should be the correct move in many situations rather than in the rarest of events, even without upgrades.
The only time I can see when Tzeench or Slaanesh should fortify themselves in a region with warriors is if they desperately need dial ticks and the warp stones, heroes, or nobles needed to do so are extremely super rare or for some reason inaccessible. Otherwise yes it is generally enough to make people spend their points on warriors to attack you and then just pack up and leave. (Or better yet use a more efficient card based solution if that is available to you).
I feel that starting warriors off with some minor powers could be an alternative solution to simply altering the upgrade cards. Something like, summoning a warrior from the board costs 1 power point instead of 2. Or, you may summon a warrior to any region that has warp stones or magic symbols or some such. An example of a more interesting upgrade might be something that affords a bit of actual protection for your cultists... Each Daemonette protects one cultist. That might be too strong in combination with the current seductress upgrade, although if balanced could give Slaanesh a reasonable shot at dial advancements in games with poor setup for him/her.