Rifts d20... yeah, this has come up more than a few times over the past 10 years.
So a lot of folks will talk about how it's hard to do a conversion because Palladium is so rabid. *shrug* Palladium's approach to things doesn't make it _easier_ I grant, but that's not really the problem.
The first problem is, What is Rifts?
No, it's not a koan, it's an honest question for you to think about. See, different people are going to hook on to different things; that might sound obvious, but you'd be amazed at how many people look at the same game, play by the same rules, and feel the game is about wildly different things.
Some folks might dig exploring the setting, some folks are going to dig the crazy classes (and Min-Maxing them), some folks are going to see a goofy-over-the-top-turned-up-to-11 setting, others are going to see a harsh world with a mis-matched rules system... are you catching my drift here?
Ok, you hate the Palladium rules. Old news. What is it about the _new_ rules that you like? Is it because it's "balanced"? Is it because it's popular? (don't knock it, lots of folks play a game because it's popular; it means it's easy to find players/GMs) Is it because you already basically know them and don't feel like learning another set? (another popular reason for conversions) Is it because you're cheap and don't want to spend money buying new things, just using what you've already got?
Answers to these are kinda important, because if you're not willing to spend money, it's going to limit your options. If it's a system mastery issue, that might limit them in other directions. If it's an issue of balance, that's going to change suggestions as well.
For example, I'm pretty confident I could knock together a more than respectable version of Rifts d20 using: BESMd20, Advanced Magic, and Mecha Compendium Deluxe (Dream Pod 9, but uses GoO's Mecha SRD as the base). Those are all BESMd20-based products.
Flipping it a different way, I'm reasonably confident I could work out a Rifts d20 using Silver Age Sentinels d20. It's pulling from the same BESMd20 base, but since it's a supers game it's got a bit of a different focus.
Mutants & Masterminds should be able to do Rifts d20 as well; there's hordes of M&M fans out there that'll tell you how M&M can run any game there is.
But let's put that aside... you've mentioned Star Wars Saga (a game I'm not really familiar with) as the core and then you're adding on to it from there.
First thing you might do is stop over here:
Sword and Sorcery Saga, by John "The Gneech" Robey
From there you can download a Sword & Sorcery conversion, which allows you to run S&S style games using SWSE. It's not directly applicable to what you're doing, but since it's using traditional fantasy classes, it might help you figure out a consistent approach for doing your class building. Like figuring out what base class to use for what and assigning Talent Trees to refine the class from there.
The other thing you might take a look at while you're at Robey's site is the Saga Edition Encounter Budget. It might not be an exact sort of thing, but it's probably going to help you out.
What about vehicles and power armour and Glitterboys?
Yeesh. Well, d20's focus on "balance" and Palladium's lack of caring for it really make this tricky. You might be able to bash something together with SWSE. I dunno.
If that stuff is going to be a major influence in your game, you might consider messing around with the Mecha SRD. It's intended to work for regular d20 so you'll probably still need to massage things a bit, but it'd be a way of consistently building all the gear.
Guardians of Order are long gone but you can still get the Mecha SRD as well as the Anime SRD here:
Guardians Of Order - d20 - Open Game Content System Reference Documents
The Mecha SRD is going to be a decent enough starting point for figuring out how to handle the Glitterboy, not just the mech itself, but the class as well.
You can get a more "human readable" version of the Anime SRD here:
FateRPG.Com: Fate: Fantastic Adventures in Tabletop Entertainment (TM)
I mention this because the Anime SRD might be of use to you as well. Why? Well, there's that pesky issue you've (potentially) got of... Dragons. Or just about any other funky race. How? Anime SRD (the SRD version of BESMd20) is a points-based system. Now, some folks would like to argue about how "accurate" the point system is and all that, but it _really_ doesn't matter as much as some folks would like to think.
For your purposes, you basically need to figure out a rough equivalency. About how many Character Points are the other classes getting to spend to do their thing? Probably a bit hard to figure out isn't it?
Well, a potential solution for you is to grab the Adventurer class out of the Anime SRD. See, the various classes in the Anime SRD are all basically equivalent to each other. The nice thing about the Adventurer is it's essentially a blank slate. It provides very basic progression in terms of BaB and Saves. Most importantly is the "5 Character Points" which are gained each level.
You can tweak around the base progressions of the Adventurer class to suit whatever your sensibilities. Depending on how you tweak it (if at all) and what Attributes the dragon (or whatever) can buy, you're going to have a decent idea of the power of the character.
The player basically can choose as he levels up if he wants to pimp out the dragon with dragon abilities, or whatever.
I dunno if any of the above is useful, but hopefully it at least provides some food for thought.
Someday, someone is going to get around to doing a clone of Rifts, very very very carefully relying on Open Game Content to do so, while avoiding mentioning Palladium I.P. Sometimes I think that person might be me, other times I think "Bah. Too much effort." I guess time will tell.