Ken:
If you want to be able to fire factory ammo, that will limit you. Here are a few. I'm sure others will offer others. The easiest ones are where the ejector does not need replacement/modifications. IE: rimed to rimed or rimless to rimless.
22 Hornet > 221 Fireball
22 Hornet > 225 Winchester (This is a good one. Did it. Works great.)
357 Mag > 357 Max
357 Mag > 35 Remington
30-30 Win > 30-40 Krag
30-30 Win > 30-30 AI (Controversy aside, this one works great. Its the same as rechambering to a 30-40 Krag. It headspaces on the rim like the Krag, and has NO crush fit isssue. And you can still fire factory 30-30s.)
7.62 x 39 > 7.7 Japanese, 303 British or 7.65x53
44 Mag > 445 Mag or 444 Marlin
Hope this helps you get started. There is a lot of fun in shooting a unique caliber.
Wally