I would expect a more conservative remaster of Tellius than a full-blown Echoes-style remake.
Flesh out parts of RD's story more, hopefully add full supports. Classic supports could be combined with the everyone-supports-everyone system, just give some combinations full conversations. Perhaps even a handful of new chapters, like Part 1 could slow down the pace of the war a bit.
Touch up the graphics. RD is pretty good-looking already; if you bring PoR up to its level you only need minor upgrades on top of that I think.
Add voice acting, re-record cutscene VA.
Add some of the features and QOL in newer games, such as casual mode. RD hard needs enemy ranges back too.
Tweak the balance a little (earth supports should be nerfed, for one). Perhaps even a new difficulty level for hardcore fans.
Fix localization issues - PoR maniac mode, RD's difficulty naming and extended script.
That wouldn't be that difficult to do, could be sold full price if both games were bundled together, and would probably sell well to a newly expanded audience.
Tellius had pretty bad timing for sales and deserves a second chance, with PoR coming on a basically dead system and RD on a successful system but sequel to a game few played and a niche game that didn't really fit the Wii's audience early on. Clearly there is some demand there for the prices they're going at. Awhile back I wondered if the fact that RD was one of the few first-party Wii games not to be released on Wii U eShop (Twilight Princess was another) meant they were planning to do something with it.