
Professor X will help Sentry overcome Void (at least temporarily)
One theory I actually think could work really well is Professor X helping Bob (Sentry) control the Void during the Sentinel battle.
Bob's biggest limitation is his inability to safely use Sentry without risking the Void taking over. Charles could enter Bob's mind and help him gain control over it.
There is even comic precedent for this, i.e Emm Frost entering Sentry's mind and helping him contain the Void.