I've been recommending that pioneers separate city/outpost construction since pre-1.0.

The approach I took was to have the Scout unit be consumed when an outpost is constructed. This gives the player a choice: Either explore further, or set up camp. It also gives the Scout unit validity as currently there is none since you have heroes to explore territory.

The pioneer could then undergo any change needed to solve city spam.

@Ambermonk: Very interesting idea. I would argue however that the Engineer unit would/should still be consumed when building an outpost. Shouldn't make them too cheap. I like how you've added that the land may need to be 'prepped' before a city foundation can be laid. Excellant idea!

You can build your own engineering units if you want. Here is what i've modded for myself. Basically, it offers you 2 more traits to choose when you're designing units.

    <AbilityBonus InternalName="Settler_Unit_Bonus">
        <AbilityBonusType>Unit_Design</AbilityBonusType>
        <AbilityBonusOption InternalName="Settler_Unit_Bonus">
            <DisplayName>Settler</DisplayName>
            <Description>Unit can settle a new village.</Description>
            <Icon>Ability_Civics_Icon.png</Icon>
        <GameModifier>
            <ModType>Unit</ModType>
            <Attribute>UnlockAction</Attribute>
            <StrVal>BuildTown</StrVal>
        </GameModifier>
        <AdditionalTrainingTurns>65</AdditionalTrainingTurns>
        <HideWhenUpgraded>0</HideWhenUpgraded>
        <Type>Weakness</Type>
        <AIData AIPersonality="AI_General">
                <AIPriority>5</AIPriority>
         </AIData>
      </AbilityBonusOption>
    </AbilityBonus>    
        
    <AbilityBonus InternalName="Builder_Unit_Bonus">
      <AbilityBonusType>Unit_Design</AbilityBonusType>
        <AbilityBonusOption InternalName="Builder_Unit_Bonus">
            <DisplayName>Builder</DisplayName>
            <Description>Unit can build an outpost.</Description>
            <Icon>Ability_AdventurersBoon_Icon.png</Icon>
        <GameModifier>
            <ModType>Unit</ModType>
            <Attribute>UnlockAction</Attribute>
            <StrVal>BuildImprovement</StrVal>
            <StrVal2>Outpost</StrVal2>
        </GameModifier>
        <AdditionalTrainingTurns>45</AdditionalTrainingTurns>
        <HideWhenUpgraded>0</HideWhenUpgraded>
        <Type>Weakness</Type>
        <AIData AIPersonality="AI_General">
                <AIPriority>5</AIPriority>
         </AIData>
      </AbilityBonusOption>
    </AbilityBonus>

It's not perfect but it works very well. You want a fast built "engineer" ? Then build a Pioneer.

But if you want to save your population, you'll be able to build expensive units, eventually mounting horses, wearing weapons, etc. And, thanks to DsRaider for his excellent AddFactionTraits, if you have Dogs of War faction trait, you cant recruit ... mercenaries that build outposts or settle villages ! (of course, you won't design a unit having both Builder and Settler traits ... because unit will disappear once you've clicked either "Settle" or "Outpost" button)

Oh, and for the roads ... I've not tested it but you could try this one :

    <AbilityBonus InternalName="RoadBuilding_Unit_Trait">
        <AbilityBonusType>Unit_Design</AbilityBonusType>
        <AbilityBonusOption InternalName="RoadBuilding_Unit_Trait">
            <DisplayName>Road Building</DisplayName>
            <Description>Allows the unit to build roads and crush scorpions</Description>
            <Icon>Ability_PathOfTheGovernor_Icon.png</Icon>
            <GameModifier>
                <ModType>Unit</ModType>
                <Attribute>UnlockAction</Attribute>
                <StrVal>BuildRoad</StrVal>
                <Provides>Allows the unit to build roads</Provides>
            </GameModifier>
        <AdditionalTrainingTurns>45</AdditionalTrainingTurns>
        <HideWhenUpgraded>0</HideWhenUpgraded>

            <Type>Weakness</Type>
            <AIData AIPersonality="AI_General">
                <AIPriority>5</AIPriority>
            </AIData>
        </AbilityBonusOption>
    </AbilityBonus>

One idea I have on this that goes with the "Build multiple outposts, but once you build one you can't build a city" is...

The Pioneer unit has 3 people in it.  How about put a requirement on the Settle ability that costs all 3 people, and put a requirement on the Outpost ability that only costs 1 of them.  After you build the first Outpost, you won't have enough people left in your Pioneer unit to Settle, but you can build two more Outposts.

This will keep the Pioneer as the unit who does both, but also give purpose to it being 3 people.  It also means that if you're attacked and one of the people in the Pioneer unit is killed, you won't be able to Settle until that person heals, either over time or from a spell.  Naturally, the healing recovery wouldn't happen from those you lost from building an Outpost.  Of course, that would be a rare situation.

Looking at these posts, see some great ideas here. 

I would go with the idea of Pioneers making up to three outposts with a population of 10 people each; also give Pioneers the ability to raise and lower land (with some sort of prerequisites). maybe also have a mechanism where Pioneers could be absorbed by a city to add to the population. This would be a handy way to grow a newly founded city.

I also support the idea of separate build lines for units and buildings; this is something I really hated in CivIV. That way you can train units without suspending construction of buildings.

Outposts should have maybe a Garrison (Militia) unit for defense (maybe with the ability to upgrade weapons and armor? I would like to see this for cities too).

Outposts should also have some limited build options; since their population is "fixed", they should build some limited upgrades. Increased defenses, accelerated healing for units etc.

Ambermonk I attempted to add a build queue just as you suggest but could not get it to work.  That is not saying it is not possible but just that it has been attempted with no success so far.  I basically attempted to add that it was a cityhub tag and then limit it to level 1 while maintaining it as an outpost but changing the display name to Hamlet.

I don't think we should go too crazy with the idea of Engineers.  Building roads removes most of the point of having a Governor Champion (although, really, who chooses that path?).  Allowing a unit to raise and lower land removes some of the point of specializing in Earth magic (where there's a spell that does that).  Allowing for infinite Outposts built by a single unit would remove a lot of the point of choosing the The Decalon faction strength (even my suggestion would reduce the point in getting that).

I do like the idea of the outpost having its own build queue for its own upgrades.  Especially considering they currently cost the same as a city to build.  Also, a separate unit whose sole purpose is creating outposts, and costs less population (as suggested in the OP), is also good.

I use the faction trait 'Master Scouts' which allow all of your units to move at a normal rate across much of the terrain, and, of course, doesn't have a negative effect on your building queue.