It's way too early to even know for sure what we have in Wolanin and Lajoie. So, I'm assuming if the team values Ceci enough to sign him to a long term extension at top 4 money, that they will value him enough to want to protect him.
By acquiring another top 4 defender in that scenario, it's an absolute waste. That's all I'm saying. It puts the team in a position where if any one of Wolanin, Lajoie, or Jaros take steps forward, they risk either losing that top 4 defender, losing the top 4 defender they give up assets to acquire in a trade, or losing Ceci who they presumably view as a top 4 defender.
Even if you take Ceci out of the equation, it's still a risk to move assets for a defender right now because it creates a scenario where if two of Lajoie, Wolanin, or Jaros develop into a #4 or better, we've then traded assets to rent an extra top 4 defender for two years, and then hand Vegas and much more valuable player than we'd otherwise have had to expose.
I don't know who I'd protect right now other than that Chabot is for sure protected. I probably would not sign Ceci to a 4-5 year deal at top 4 or better money right now, if that's the Senators plan. Although, it's not entirely relevant what I'd do, since the idea that acquiring a top 4 defender doesn't make sense because of expansion was predicated on the idea that the Senators value and would re-sign/protect Ceci. There's a reasonable chance at least one of Wolanin, Lajoie, or Jaros will be worth protecting 2 years from now. Add that to Ceci+Chabot, and it makes it difficult to square up acquiring a top 4 defender with the expansion protection lists.