The banner change is pretty boring. While it does increase their effectiveness, the solution doesn't address how mechanically uninteresting they are. If anything it makes it worse by turn them into aoe signets, minus the active.
I think proposed solutions should center around the banner's theme of area control. Right now that focuses on ally enhancement. But it could also, for example, cripple foes that leave the banner's area.
Mechanically this can be achieved by giving a debuff to enemies, and when the debuff expires they are cripple for several seconds. The debuff could be the same across all banners in order to reduce effect bloat. Could call it something like: "Banner's Challenge" Target is crippled for X sec when effect expires.
This would help keeps foes inside the effective range of the banner, would trigger even on foes who teleport through the threshold, encumber fleeing foes (help chase), give control over the effect's activation to the warrior (picking up the banner), and has good synergy with the typically close range style of most warrior builds and cripple traits.
The only thing to note is that there would be a short delay between leaving the banner and the effect expiring. I dont see that as a big deal though.