Note that the sanctuary space isn’t really a traditional or a preferred location for the baptismal font. I would assume this is especially true in the (sometimes lamentable) situation of portable or movable fonts. My last parish, despite having a strong liturgist as a pastor, could never muster the energy for a permanent font, let alone wheel the portable one to a location outside the sanctuary.
That said, there are positive values to consider, so let’s get to them:
§ 67 § The location of the baptismal font, its design, and the materials used for its construction are important considerations in the planning and design of the building. It is customary to locate the baptismal font either in a special area within the main body of the church or in a separate baptistry. Through the waters of baptism the faithful enter the life of Christ.(RCIA 213) For this reason the font should be visible and accessible to all who enter the church building. While the baptistry is proportioned to the building itself and should be able to hold a good number of people, its actual size will be determined by the needs of the local community.
If a font is placed in the nave, then the standards of a good baptistry, especially design and room for people, should be guiding the planning and discernment. How many are a “good number of people”? That would depend on the number of those attending a baptism outside of Mass, routinely. A separate room should be able to provide for thirty at minimum. A location near the juncture of nave and narthex with flexible seating might be optimal in many ways.
All texts from Built of Living Stones are copyright © 2000, United States Conference of Catholic Bishops, Inc. All rights reserved. Used with permission.
Catholic Sensibility 
A separate baptistery (or a baptistery under the vault of a bell-tower in or adjacent to the narthex) is where I’d like to see domes and jewel-box vault work, architecturally speaking (as opposed to other places….); a complementing Eucharistic chapel would also work well (while you could site a baptistery under a tower in the center of a narthex, it would not be prudent to so site a Eucharistic chapel – rather, if one wants to pair those spaces, you would site them at either end of the narthex). Yes to mosaics on the vault and walls. Commissioned over generations, of course. And bells, likewise. A church to me is incomplete without at least one real and externally audible bell; bells should be envisioned as part of the work of generations.