Make sure you understand the difference between "registering" a domain-name, provisioning domain-name service for it, and actually running a mailserver or hosting content in it.
It may be that you're paying for a package that includes domain-name registration, nameserving, webhosting, and mail, and you could save money by dealing with a different registrar for the registration, using a service like DynDNS or Amazon Route 53 for nameserving, and running a forwarding-only email service on the old domain. Whatever hosting provided you use for the
.com domain can probably accept e-mail for additional domains, or you could using something like Amazon SES, currently free for the first 1000 incoming e-mail messages per month.
And the registrar may offer you a chance to lock the current rate in for 5 years, not just another year. That should give you time for a long-term plan as well.
But some of the newer gTLDs are more expensive to register or reregister a domain in. According to Compare Prices of All Top-Level Domains, the most-expensive TLD is currently
.hoteles, about $65000 to register and about $71500 to renew. If you have an important personal or business e-mail address in a vanity TLD like that, you're tied down to it. However, if you have a really valuable domain (in that TLD, or anywhere), you might monetize it by selling it, but make sure the contract specifies that the new owner (and any subsequent assignees) will forward and not store e-mail sent to certain addresses, during some specified period.