WELS and ELS already are, actually. They broke fellowship with the LCMS in the 50’s and 60’s for fear (rightfully) of the LCMS’s sizable Liberal faction which practiced Historical Criticism. Most of that sort left in the 70’s to lay the groundwork for what would become the ELCA in the 80’s. That left the orthodox Confessionals in Missouri - but the damage had been done. WELS and ELS had already broken fellowship with the LCMS, and were hesitant to believe Missouri had righted its ship.
Since those turbulent times, the twin synods established somewhat reactionary and certainly more conservative practices to safeguard against suffering a Missouri-like schism. One example is that WELS and ELS, unlike LCMS, will not even pray with other Christians, because it could appear as if they condone sycretism or unionism. There also exists some differences in how the groups understand the pastoral office - Missouri says Christ specifically instituted the pastoral office, while the twins say it’s simply a special manifestation of the general priesthood of all believers. Naturally, Missouri considers that a departure from the Lutheran Confessions and Holy Scripture. There’s still some work to do, but convergence will be found.