I'd suggest New Desides, personally. Much, much less explored set of antagonists and a more interesting foil to Londo Bell. Though they would either be a very small group, or a group that splintered off from Neo Zeon since the New Desides either died or joined Axis Zeon at the end of Sentinel.

But a group of New Desides and Titans remnants operating their signature machines mixed in with some Zeonic designs would be both plausible and interesting.