One issue that I believe needs to be resolved is what ruleset is UP going to use-the standard D&D rules, D20 Modern/Urban Arcana, or a mix of the two?
For those that don't know, here are the changes in D20 Modern in summary:
Base and Advanced Classes: rather than use a multitude of 20-level core classes, D20 Modern has 6 core classes, one for each ability score, that feature 10 levels of advancement. The core classes offer a choice of bonus feats and class abilities.
Advanced Classes are a step above the 6 core classes, giving PCs a higher degree of focus in their role. Unlike prestige classes, Advanced classes are easier to enter, and can be entered as early as level 3 or 4.
Action Points: These work in the same way as described in Unearthed Arcana and Eberron. For UP, these points can be used to give a tangible example of belief shaping reality on the Planes.
Lower Damage Threshold: In D20 Modern, damage that goes above one's Constitution score forces a save against death by massive damage. This allows modern firearms to remain deadly at high levels. Note that in some places (such as Ysgard), this rule might be waived.
Low Magic Setting: Even for Urban Arcana, magic above the 5th level is restricted to Incantations. Perhaps the secrets of the Art were lost at one point, and the powers are no longer as willing to share them. Regardless, this would require a rebalancing of planar beings on the planes.
I voted pure d20 modern. It is an action-packed system, but finally one where level 15 players don't just treleport around and blow up cities with fireballs 
The only two usable dnd elements i can think of would be:
1) gold pieces or other currency - you can't haggle with DC costs
2) Lots of MM creatures would probably still be there, like baatezu or slaadi :twisted:
Also, don't let "modern only" votes stop you from going with what you feel like. If you want a dude who rides a horse and has a lance (think: don quixote), you shouldn't just say "no! We said modern only! I'm not letting any spirited charges into my setting!!!1". Choose wisely but choose freely
Hey, I think that's a Speed Demon build. Maybe a Tiefling or Half-Fiend Speed Demon? (Pun intended)
I voted for a mix, cause I like me my magic. 
I'd say we use d20 Modern for everything but class structure and seriously upping the availability of magic. I believe it isn't really Planescape without spellslingers and Proxies.
For classes, my suggestion is we just integrate the Sorcerer, Wizard, Cleric, Druid and possibly Bard classes (and maybe some magical Prestige classes) from D&D, and have the rest just be d20 Modern with stiffer penalties for multiclassing with one of the "older" classes. (Characters could be said to be more flexible in modern times, but the old paths to power still exist, if they are somewhat rigid.) Alternately, we could just integrate a modified version of the Spellcaster generic class from Unearthed Arcana.
Thanks
Luc "I likes me some magic" French
I like a near-pure d20 Modern, for the simple sake of maximizing rules compatibility. It also has the bonus of offering a lower-power alternate ruleset for players who like standard Planescape, but don't want the availability of magic to be as high.
While I agree with you, I think that a way to access higher level magic should be provided, given the results as they stand. There's already incantations in Urban Arcana, so mabye we can allow for even higher level magic through this path.
The problem, as I see it, is that the more prevelant high-magic is, the more it has to be balanced with the D20 Modern rules. This is especially tough because there are major balance issues with 3.5 magic as it stands.
Also, Adding the 3.5 spellcasting classes alongside the "normal" D20 Modern classes effectively segregates PCs and NPCs into two distinct groups. In my opinion, this will inevitably lead to severe balance issues that would ultimately favor one group or the other. Better to decide on one class structure and stick with it.
Compromise Proposal-Currently, spellcasting prestige classes in UA allow up to 5th level. Those who have reach 5th level spellcasting can access another prestige class (one with a good deal of roleplaying requirements, such as belonging to a certain faction or allegience to a certain power) that would give normal progression to 7th level spellcasting, and open the door for using Incantations and/or "Spell Secrets" to cast 8th and 9th level spells.
I actually think that Incantations can lend a lot of flavor to the setting, and make high-powered magic feel 'special'.
:shock: I feel like persuading someone into running d20 modern. Man, I'd love to try a horse-riding speed demon :twisted:
As far as I've seen in SRD, there are guidelines to make your own incantations. So, in dire need of spells like "genesis", I don't think it should be hard to cook something up with incantations
I think so far using d20 modern just accounting for the increase in spell power sounds best.
That's really what I preferred when I said near-pure. Personally, I'm of the opinion that most of the really big effect spells/psionic powers SHOULD be incantations.
Turning some of the big 6th-9th level spells into incantations while leaving some more combat-oriented spells as quick spells might be an interesting way of taking a middle road between the D&D3.5 rules and pure D20Modern rules, and offer a way of allowing power rebalancing to happen more naturally.
Yup. If memory serves, Advanced classes are just like Prestige Classes in Modern. They're quite good too.
For instance there's Soldier, Bodyguard, Martial Artist, Field Medic and many others.
I've also got the image that d20 Modern classes are more balanced than some of D&D's. Nevertheless, I voted for combining Modern and standard D&D.
There are multiple versions of Advanced classes right? Allowing the focus of a character (much like a PrClass?)