Don't really see Transnistria as remotely useful as a bargaining chip. Sure it's propped up by Russia and would eventually be integrated into Russia under Putin's wildest imperial fantasies, but it's not Russia, its people mostly aren't Russian and I'm not sure it's all that valuable either.

All a Ukrainian invasion of Transnistria would do is give Putin's claims about Ukrainian aggression some veneer of legitimacy and annoy Moldova, which doesn't want to see something which is constitutionally part of their territory fought over and then bargained back to Russian control...